Method of making a gyricon display using magnetic latching

ABSTRACT

A method of making rotating element sheet material utilizing magnetic latching is provided. The first step is providing a sheet of sheet material comprising a substrate with a surface and rotatable elements disposed therein. The elements each have an optical and electrical anisotropy, and comprise at least two portions. One of the portions is magnetizable. After the magnetizable portions have been oriented in a common direction, they are magnetized substantially uniformly. The elements can be oriented such that the magnetized portions are disposed towards the surface of the substrate. A layer of hardenable mixture containing a plurality of magnetic particles is then applied to the surface of the substrate. The hardenable liquid is kept liquid for a period of time to allow the migration magnetic particles to the area of the layer in the vicinity of the magnetized portions of the rotatable elements. Then the hardenable liquid is solidified to trap the magnetic particles in said layer in the area of the layer in the vicinity of the magnetized portion of the rotatable elements to form a magnetic pad.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to the following U.S. patentapplications that are filed simultaneously with this application:

"Gyricon Displays Utilizing Magnetic Addressing And Latching Mechanisms"by Sheridon, U.S. patent application Ser. No. 09/199,544,

"Gyricon Displays Utilizing Rotating Elements And Magnetic Latching" bySheridon, U.S. patent application Ser. No. 09/199,403,

"Gyricon Displays Utilizing Magnetic Elements And Magnetic Trapping" bySheridon, U.S. patent application Ser. No. 09/200,555, U.S. Pat. No.5,992,256,

"Method Of Making Uniformly Magnetized Elements For A Gyricon Display"by Sheridon, U.S. patent application Ser. No. 09/199,646,

"An Apparatus For Making Uniformly Magnetized Elements For A GyriconDisplay" by Sheridon, U.S. patent application Ser. No. 09/200,406,

"A Method Of Making A Gyricon Display Using Magnetic Latching" bySheridon, U.S. patent application Ser. No. 09/199,818,

"A Method Of Making A Gyricon Display Using Magnetic Latching" bySheridon, U.S. patent application Ser. No. 09/199,543,

"Magnetic Unlatching And Addressing Of A Gyricon Display" by Sheridon,U.S. patent application Ser. No. 09/199,473,

INCORPORATION BY REFERENCE

The following U.S. patents are herein fully incorporated by reference:

U.S. Pat. No. 4,126,854 by Sheridon titled "Twisting Ball PanelDisplay",

U.S. Pat. No. 4,143,103 by Sheridon titled "Method Of Making A TwistingBall Panel Display",

U.S. Pat. No. 5,262,098 by Crowley et al. titled "Method And ApparatusFor Fabrication Bichromal Balls For A Twisting Ball Display",

U.S. Pat. No. 5,344,594, by Sheridon titled "Method For Fabrication OfMulticolored Balls For A Twisting Ball Display",

U.S. Pat. No. 5,389,945, by Sheridon titled "Writing System IncludingPaper-Like Digitally Addressed Media and Addressing Device Therefor"

U.S. Pat. No. 5,604,027 by Sheridon titled "Some Uses OfMicroencapsulation For Electric Paper",

U.S. Pat. No. 5,717,514 by Sheridon titled "Polychromal Segmented BallsFor A Twisting Ball Display",

U.S. Pat. No. 5,767,826 by Sheridon et al. titled "Substractive ColorTwisting Ball Display",

U.S. Pat. No. 5,777,782 by Sheridon titled "Auxiliary Optics For ATwisting Ball Display",

U.S. patent application Ser. No. 08/716,672 (D/925105) by Sheridon etal. titled "Twisting Cylinder Display",

U.S. patent application Ser. No. 08/960,865 (D/92105i) by Sheridon etal. titled "Twisting Cylinder Display",

U.S. patent application Ser. No. 08/960,868 (D/92105q1) by Sheridontitled "Twisting Cylinder Display Using Multiple Chromatic Values", and

U.S. patent application Ser. No. 09/960,868 (D/98095) by Howard et al.titled "Charge Retention Islands For Electric Paper And ApplicationsThereof".

BACKGROUND

This invention relates generally to Electric Paper or Gyricons and moreparticularly concerns a rotating element sheet material in whichmagnetic fields are used in addition to electric fields for addressing,latching the rotating elements into place once an image has beenselected for display, and to provide selected threshold behaviors forindividual types of elements.

Lee (L. L. Lee, "A Magnetic Particles Display", IEEE Trans. On Elect.Devices, Vol. ED-22, Number 9, September 1975 and L. L. Lee, "MatrixAddressed Magnetic Particles Display", in 1977 Soc. For InformationDisplay International Symposium, Digest of Technical Papers, Boston,April 1977) has described the addressing of a twisting rotating elementdisplay in which the rotating elements have a magnetic dipole withmagnetic fields. U.S. Pat. No. 3,036,388 by Tate, and issued in May 1962uses a stylus consisting of a magnetic dipole to address a displayconsisting of magnetized particles having black and white surfacescorresponding to a given magnetic polarity. More recently, U.S. Pat. No.5,411,398 by Nakanishi et al. and titled "Magnetic Display System"describes the use of a magnetic dipole to address a display consistingof black ferromagnetic particles and white, non-magnetic particlesdispersed in an oil and in turn contained in microcapsules arranged in alayer. Upon application of a magnetic dipole, the black ferromagneticparticles are pushed to the rear of the microcapsules, revealing onlythe white particles, or pulled to the front of the microcapsules so thatmostly only the black ferromagnetic particles can be seen by anobserver.

In the above prior art only magnetic fields are used to address twistingor moving magnetic particles and rotating elements. There is no mentionor attempt to use electrical fields combined with magnetic fields.

U.S. Pat. No. 4,126,854 titled "Twisting Ball Panel Display" issued Nov.21, 1978, and U.S. Pat. No. 4,143,103 titled "Method Of Making ATwisting Ball Display", issued Mar. 6, 1979, both by Sheridon, describea twisting rotating element (or "Gyricon") display that comprisesbichromal rotating elements contained in liquid-filled sphericalcavities and embedded in an elastomer medium. One segment of thebichromal rotating elements has a larger electrical charge in contactwith the liquid and in the presence of the electrical field than theother segment. Thus, for a given polarity of applied electrical field,one segment will rotate toward and be visible to an observer of thedisplay. Applying the opposite polarity of electrical field will causethe rotating element to rotate and present the other segment to be seenby the observer.

U.S. Pat. No. 4,143,103 describes the response of the bichromal rotatingelement to the applied electrical field as a threshold response. Thatis, as the external field is increased, the bichromal rotating elementremains stationary in position, until a threshold voltage is reached, atwhich time the rotating element starts to rotate from its initialposition. The amount of rotation increases with an increasing electricalfield until a 180 degree rotation can be achieved. The value of theexternal field that causes a 180 degree rotation is called the fulladdressing voltage.

The response pattern of the bichromal rotating element to an externalelectrical field determines the types of addressing that may be used tocreate images on the Gyricon display. There are known in the art threetypes of addressing schemes for displays. The first of these is activematrix addressing, which places the least demands on the properties ofthe display.

In active matrix addressing a separate addressing electrode is providedfor each pixel of the display and each of these electrodes iscontinuously supplied with an addressing voltage. The complete set ofvoltages can be changed for each addressing frame. This type ofaddressing places the least demands on the properties of the displaymedium, however, active matrix addressing is the most expensive, mostcomplicated and least energy efficient type of addressing.

The second type of addressing scheme is passive matrix addressing.Passive matrix addressing makes use of two sets of electrodes, one oneach side of the display medium. Typically, one of these consists ofhorizontal conductive bars and the other consists of vertical conductivebars. The bars on the front surface or window of the display arenecessarily transparent. To address the display medium a voltage isplaced on a horizontal conductive bar and a voltage is placed on avertical conductive bar. The segment of medium located at theintersection of these two bars experiences a voltage equal to the sum ofthese two voltages. If the voltages are equal, as they usually are, thesections of medium located adjacent to the each of the bars, but not atthe intersection of the bars, experience 1/2 the voltage experienced bythe section of medium at the bar intersection. Passive addressing isless complicated and more energy efficient because the pixels of thedisplay medium are addressed only for as long as is required to changetheir optical states. However, the requirements for a medium that can beaddressed with a passive matrix display are significantly greater thanfor the active matrix case. The medium must respond fully to the fulladdressing voltage but it must not respond to 1/2 the full addressingvoltage. This is called a threshold response behavior. The medium mustalso stay in whichever optical state it has been switched into by theaddressing electrodes without the continuous application of voltage,that is it should store the image without power. Passive addressing isthe most widely used method of addressing displays and is the lowestcost.

The third type of addressing, and probably the most useful for ElectricPaper (paper surrogate) applications, consists of a linear array ofaddressing electrodes in the form of a bar that can be moved over thesurface of the display medium. Typically, the medium is placed over agrounding electrode and is protected from possible mechanical damagefrom the moving bar by placing a thin window between the bar and theElectric Paper. As the bar is moved over the display medium, it appliesvoltages to specific pixels of the medium for short periods of time andgenerates a full image each time the bar is scanned over the surface. Inone variation of this method, the addressing bar deposits image-wisecharge on the surface of the window.

The requirements imposed on the display medium by this form ofaddressing then depend on which type of addressing bar is used. If theaddressing bar simply exposes the medium to voltages as it passes overthe display window, then it is necessary for the display medium toexhibit threshold behavior. Thus the area of the medium directly underthe addressing bar electrode must change optical states when exposed tothe full addressing voltage, but as the bar moves to the next row ofpixels, this same area of medium must not respond to the diminishedvoltages experienced by the medium from the moving addressing bar. As inpassive addressing, this requires that the medium have a sharp thresholdresponse. This addressing bar also requires that the optical state ofthe medium completely change during the time the addressing barelectrodes move over its vicinity which usually limits the display frameaddressing speed. Copending U.S. patent application Ser. No. 09/037,767by Howard et al. and titled "Charge Retention Islands For Electric PaperAnd Applications Thereof" also assigned to the same assignee as thisapplication, describes an arrangement of addressing electrodes thatgreatly reduces the switching speed requirements of the medium due tothis effect.

In U.S. patent application Ser. No. 09/037,767 the addressing bardeposits image-wise charge on the surface of the display window. Thecharge deposition addressing method relaxes the requirements on thedisplay medium. The addressing bar speed over the medium surface islimited only by the rate at which it can deposit image-wise charge,because the medium can respond to the voltage associated with thedeposited charge pattern at its own speed. Threshold response behavioris not so important, however the ability to store the image is becauseit can be expected that the image-wise charge deposited on the windowsurface will leak off over a short period of time. However, addressingbars that can deposit image-wise charge on the display window tend to bebulky and more expensive than bars that simply impose image-wisevoltages directly.

There is a need, therefore, to find other means to control the opticalswitching characteristics and optical image storage characteristics ofGyricon display media. It is the purpose of this patent application todisclose new and improved means of accomplishing this by the addition ofmagnetic materials in the composition of the Gyricon rotating elementsand the sheet material and by the use of externally imposed magneticfields.

Accordingly, it is the primary aim of the invention to provide a meansfor controlling the optical switching characteristics and the imagestorage characteristics of gyricon sheets by using magnetic materialsand magnetic fields to provide sharp and uniform threshold voltages,provide improved image latching characteristics, and in conjunction withelectric fields to provide improved addressing methods.

Further advantages of the invention will become apparent as thefollowing description proceeds.

SUMMARY OF THE INVENTION

A method of making rotating element sheet material utilizing magneticlatching. The first step is providing a sheet of sheet materialcomprising a substrate with a surface and rotatable elements disposedtherein. The elements each have an optical and electrical anisotropy,and comprise at least two portions. One of the portions is magnetizable.After the magnetizable portions have been oriented in a commondirection, they are magnetized substantially uniformly. The elements canbe oriented such that the magnetized portions are disposed towards thesurface of the substrate. A layer of hardenable mixture containing aplurality of magnetic particles is then applied to the surface of thesubstrate. The hardenable liquid is kept liquid for a period of time toallow the migration of magnetic particles to the area of the layer inthe vicinity of the magnetized portions of the rotatable elements. Thenthe hardenable liquid is solidified to trap the magnetic particles insaid layer in the area of the layer in the vicinity of the magnetizedportion of the rotatable elements to form a magnetic pad.

BRIEF DESCRIPTION OF THE DRAWINGS

While the present invention will be described in connection with apreferred embodiment and method of use, it will be understood that it isnot intended to limit the invention to that embodiment/procedure. On thecontrary, it is intended to cover all alternatives, modifications andequivalents as may be included within the spirit and scope of theinvention as defined by the appended claims.

FIG. 1 shows a cross-sectional view of a prior art gyricon.

FIG. 2 shows a cross-sectional view of a prior art gyricon.

FIG. 3 shows a perspective view of a prior art gyricon.

FIG. 4 shows a cross-sectional view of a first embodiment of a gyriconsheet according to the present invention.

FIG. 5 shows a cross-sectional view of a method of making rotatingelements according to the present invention.

FIG. 6 shows a cross-sectional view of an apparatus used to makerotating elements according to the present invention.

FIG. 7 shows a first step in a process used to make a gyricon sheetshown in FIG. 4.

FIG. 8 shows a second step in a process used to make a gyricon sheetshown in FIG. 4.

FIG. 9 shows a third step in a process used to make a gyricon sheetshown in FIG. 4.

FIG. 10 shows a first step in an alternate process used to make agyricon sheet shown in FIG. 4.

FIG. 11 shows a second step in an alternate process used to make agyricon sheet shown in FIG. 4.

FIG. 12 shows a first step in another alternate process used to make agyricon sheet shown in FIG. 4.

FIG. 13 shows a cross-sectional view of a second embodiment of a gyriconsheet according to the present invention.

FIG. 14 shows a process used to make a gyricon sheet shown in FIG. 13.

FIG. 15 shows a cross-sectional view of a third embodiment of a gyriconsheet according to the present invention.

FIG. 16 shows a cross-sectional view of a method of making rotatingelements according to the present invention.

FIG. 17 shows a cross-sectional view of an apparatus used to makerotating elements according to the present invention.

FIG. 17a shows an alternative embodiment to a rotating element with twomagnetic segments.

FIG. 17b shows an alternative embodiment to a rotating element with onemagnetic segment.

FIG. 18 shows a cross-sectional view of a fourth embodiment of a gyriconsheet according to the present invention.

FIG. 19 shows a cross-sectional view of a fifth embodiment of a gyriconsheet according to the present invention.

FIG. 20 shows a cross-sectional view of a sixth embodiment of a gyriconsheet according to the present invention with a rotating element in afirst orientation.

FIG. 21 shows a cross-sectional view of a sixth embodiment of a gyriconsheet according to the present invention with a rotating element in asecond orientation.

FIG. 22 shows a cross-sectional view of a seventh embodiment of agyricon sheet according to the present invention.

FIG. 23 shows a cross-sectional view of a eighth embodiment of a gyriconsheet according to the present invention.

FIG. 24 shows a first step in a process used to make a gyricon sheetshown in FIG. 23.

FIG. 25 shows a second step in a process used to make a gyricon sheetshown in FIG. 23.

FIG. 26 shows a cross-sectional view of a ninth embodiment of a gyriconsheet according to the present invention.

FIG. 27 shows a cross-sectional view of a tenth embodiment of a gyriconsheet according to the present invention.

FIG. 28 shows a cross-sectional view of a eleventh embodiment of agyricon sheet according to the present invention with a rotating elementin a first orientation.

FIG. 29 shows a cross-sectional view of a eleventh embodiment of agyricon sheet according to the present invention with a rotating elementin a second orientation.

FIG. 30 shows a cross-sectional view of a twelfth embodiment of agyricon sheet according to the present invention with a rotating elementin a first orientation.

FIG. 31 shows a cross-sectional view of a twelfth embodiment of agyricon sheet according to the present invention with a rotating elementin a second orientation.

FIG. 32 shows a step in a process used to make a gyricon sheet shown ineither FIGS. 28 and 29 or FIGS. 31 and 32.

FIG. 33 shows a cross-sectional view of a prior art gyricon.

FIG. 34 shows a cross-sectional view of a thirteenth embodiment of agyricon sheet according to the present invention.

FIG. 35 shows a cross-sectional view of a fourteenth embodiment of agyricon sheet according to the present invention.

FIG. 36 shows a cross-sectional view of a magnetic model of a gyriconsheet shown in any of FIGS. 20-24.

FIG. 37 shows a cross-sectional view of a magnetic model of a gyriconsheet shown in any of FIGS. 20-24.

FIG. 38 shows the cross-sectional view shown in FIG. 37 with anadditional magnet added.

FIG. 39 shows a first step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 40 shows a second step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 41 shows a third step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 42 shows a fourth step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 43 shows a first step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 44 shows a second step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 45 shows a third step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 46 shows a fourth step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 47 shows a first step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 48 shows a second step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 49 shows a third step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 50 shows a fourth step in an addressing process, according to thepresent invention, for a gyricon sheet shown in FIGS. 20-24.

FIG. 51 shows a cross-section of a gyricon sheet shown in FIGS. 20-24after it has been addressed according to the process shown in FIGS.47-50.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to FIG. 1 a prior art Gyricon sheet 10 is shown. The gyriconsheet consists of spherically symmetric rotating elements 12 withanisotropic electrical and optical properties. The rotating element 12can be made to rotate and thus exhibit changes in optical properties bythe imposition of external electrical fields. FIG. 1 portrays a gyriconsheet 10 as disclosed in U.S. Pat. No. 4,143,103 by Sheridon, titled"Method Of Making A Twisting Ball Panel Display", and incorporated byreference hereinabove in the form of a bichromal rotating element havingsegments 14, 16 with different electrical and optical properties. Thisrotating element 12 is located in an oil filled cavity 18 in atransparent optical medium 20. When voltages are applied to addressingelectrodes (not shown) the rotating element 12 will rotate, presentingeither the black segment 14 or the white segment 16 to the viewer.

Another method of making gyricon sheets is disclosed in U.S. Pat. No.5,604,027 titled "Some Uses Of Microencapsulation For Electric Paper" bySheridon and hereinabove incorporated by reference. The resultantstructure is shown in FIG. 2 and has the same components as thestructure shown in FIG. 1, therefore the same reference numerals will beused for the same elements. The gyricon sheet 10 is composed of atransparent optical medium 20, with an oil filled cavity 18 enclosing arotating element 12. However, due to the manufacturing process a skin 19is interposed between the oil filled cavity 18 and the transparentoptical medium 20 and encloses the oil filled cavity 18.

FIG. 3 shows an example of a gyricon sheet 22 which has cylindricallysymmetric rotating elements 24 with anisotropic electrical and opticalproperties. Notice that the cross-section of a spherically orcylindrically symmetric element is the same. The rotating element 24 canalso be made to rotate and thus exhibit changes in optical properties bythe imposition of external electrical fields. FIG. 3 portrays a gyriconsheet 22 as disclosed in U.S. patent application Ser. No. 08/716,672 bySheridon et al. and titled "Twisting Cylinder Display" and hereinincorporated by reference in the form of a bichromal cylinder havingsurfaces 26, 28 with different electrical and optical properties. Thisrotating element 24 is located in an oil filled cavity 30 in atransparent optical medium 32. When voltages are applied to addressingelectrodes (not shown) the rotating element 24 will rotate, presentingeither the black surface 26 or the white surface 28 to the viewer.

While the bichromal rotating elements 12 and 24 shown in FIGS. 1 and 3,respectively, are the simplest, many other variations also exist. U.S.Pat. No. 5,717,514 by Sheridon titled "Polychromal Segmented Balls For ATwisting Ball Display" herein incorporated by reference, describesseveral variations of spherical rotating elements. U.S. patentapplication Ser. No. 08/960,868 by Sheridon titled "Twisting CylinderDisplay Using Multiple Chromatic Values" and U.S. patent applicationSer. No. 08/960,865 by Sheridon titled "Twisting Cylinder Display", bothherein incorporated by reference, describe several variations ofcylindrical rotating elements. Improvements in the ease of addressingboth spherical and cylindrical optical elements can be made by the useof magnetic materials in their construction and the use of both magneticand electrical fields in their addressing.

Soft Magnetic Material Pad Devices

The following devices all incorporate a "soft magnetic material" in theconstruction of a gyricon sheet. The term "soft magnetic material" isused to describe a magnetic material that is capable of developing astrong magnetic dipole strength while exposed to a strong externalmagnetic field, but that is not capable of retaining significant remnantmagnetism when no longer exposed to the external field. In contrast are"hard magnetic materials" which retain significant magnetism when noexternal field is present, for example a permanent magnet. Soft magneticmaterials include paramagnetic materials, ferromagnetic materials,ferromagnetic materials and supermagnetic materials, all of which may besuitable for use in the present application.

Gyricon With Single Latched State

In FIG. 4 is shown a cross-section of a portion of a magneticallyassisted Gyricon sheet 46 made from a transparent optical medium 44. Across-section of a black and white bichromal spherical or cylindricalrotating element 34 is shown in which a black magnetized segment 40 ismade from black pigments, some of which are permanently magnetizable. Itshould be noted that black and white are used here for illustrativepurposes only and any colors could be chosen. A white unmagnetizedsegment 38 is constructed from the usual materials and is notmagnetizable. This rotating element 34 is contained in an oil filledcavity 36. A soft magnetic material pad 42 is incorporated near thecavity structure of each rotating element 34 as shown in FIG. 4 andseparated from the oil filled cavity by a separation distance D_(s). Thesoft magnetic material pad 42 should preferably have a length l nosmaller than 1/4 of the rotating element diameter d. The onlyrestriction on the upper limit of the length l is that it must not be solarge as to interfere with surrounding rotating elements or their softmagnetic material pads. This will be dictated by the packing density ofthe gyricon sheet 46. Depending on the packing density, the length l ofthe soft magnetic material pad 42 can be as large as the diameter d ofthe rotating element 34 or even twice as large as the diameter d of therotating element 34 or more. The rotating element 34 is also made frommaterials that develop electrical potentials in contact with the liquidin the oil filled cavity 36 and in the presence of the electrical field,so that the two segments 38, 40 of the rotating element 34 developdifferent electrical potentials from each other.

When the black magnetized segment 40 of the rotating element 34 isadjacent to the soft magnetic material pad 42 embedded next to the oilfilled cavity 36, a strong magnetic force tends to hold the rotatingelement 34 in place. This is because the distance between the magnetizedportion of the rotating element and the soft magnetic material pad 42 isshort compared to the dimensions of the magnetized portion of therotating element, thus providing a strong magnetic field. For practicalpurposes, the separation distance D_(s) between the rotating element 34and the soft magnetic material pad 42 should be no more than thediameter d of the rotating element 34 multiplied by a factor of three.This magnetic force will cause the rotating element 34 to be attached tothe oil filled cavity 36 wall. The magnetic force will also require alarger electrical field than otherwise to cause the rotating element 34to start to rotate in the oil filled cavity 36 because the electricalfield must first overcome the magnetic force to cause the rotatingelement 34 to rotate. However, except for the increased value of theapplied electric field, the gyricon sheet 46 may be addressed by any ofthe addressing methods described herein above or known in the art. Oncethe rotating element 34 has rotated a short distance it will experiencea much reduced force from interaction with the soft magnetic materialpad 42 and the motion of the rotating element 34 will be dominated bythe applied electrical field. Therefore if a rotating element 34 isaligned in its oil filled cavity 36 in an orientation in which themagnetized segment 40 is adjacent to the soft magnetic material pad 42,a strong electrical field will be required to initiate rotation. Bycontrolling the density to be approximately uniform for all rotatingelements and controlling the type of magnetic particles of themagnetized segment 40 of the rotating element 34, the threshold value ofelectrical field required to initiate rotation can be made uniform andsharp. This is because the effects of the magnetic field on thethreshold voltage will dominate over other effects on the thresholdvoltage, for instance that of non-uniformities in size or chemicalcomposition.

When the rotating elements 34 are in the reverse orientation, that iswith the unmagnetized segment 38 near the soft magnetic material pad 42,then the rotating element 34 will be held against the oil filled cavity36 wall by the usual forces.

Rotating Element Fabrication--Magnetic Segment

The rotating element 34 can be fabricated with a modified rotating diskassembly, as described in U.S. Pat. No. 5,262,098 by Crowley et al.titled "Method And Apparatus For Fabrication Bichromal Balls For ATwisting Ball Display", and U.S. patent application Ser. No. 08/716,672by Sheridon et al. titled "Twisting Cylinder Display" and incorporatedby reference hereinabove, or other planar stream free jet type devicessuch as those disclosed in U.S. Pat. No. 5,344,594, titled "Method ForFabrication Of Multicolored Balls For A Twisting Ball Display", bySheridon and also incorporated by reference hereinabove. Also, a methodfor making bichromal spheres with a magnetic hemisphere is disclosed inU.S. Pat. No. 4,810,431 by Leidner and titled "Method Of ManufacturingPlastic Particles For A Particle Display."

To generally understand the concepts of magnetic rotating elementgeneration, FIG. 5 shows a separator member 70, having two opposedsurfaces 72, 74 connected at edge 76, over which two fine planar streams80, 82 of hardenable material are flowing. In this example, planarstream 80 contains a white pigment while planar stream 82 contains amagnetic pigment similar to that used in the manufacture of magnetictapes, such as black magnetic pigment Type 031182 by Wright Industries,Brooklyn, N.Y. either alone or in conjunction with other black pigmentsas are known in the art. The planar streams 80,82 form an outboardreservoir 84 of liquid which contains equal, side-by side, amounts ofeach liquid from each planar stream 80, 82.

A free jet 86 of liquid is formed from the reservoir 84 when the flowrate of the liquids away from the edge 76 is great enough. Methods knownin the art for creating a free jet 86 include a spinning disk assemblyand a paddle wheel assembly which are described in U.S. Pat. No.5,262,098 by Crowley et al. titled "Method And Apparatus For FabricationBichromal Balls For A Twisting Ball Display", and a jet assembly, aplanar sheet liquid sheet, and a cylindrical liquid sheet described inU.S. Pat. No. 5,344,594, titled "Method For Fabrication Of MulticoloredBalls For A Twisting Ball Display", by Sheridon, any of which may besuitably used. If low viscosity hardenable liquids are used, the freejet 86 breaks up into rotating elements 88 at its distal end as shown inFIG. 5.

While the rotating elements 88 are in flight from the free jet 86, theypass through a steady magnetic field 94, which is shown being created bytwo magnets 90, 92. As the rotating elements 88 pass through themagnetic field the section of the rotating elements 88 containing themagnetic pigment will become magnetized. As the rotating elements 88 areidentically oriented with respect to their trajectories, they will beidentically magnetized with respect to their geometric poles. The steadymagnetic field may be created by any number of ways known in the art,for example, a permanent magnet, an electromagnet, an electric field ora direct current flowing through a coil. To properly magnetize themagnetic pigment the magnetic field 94 should be at least 50 gauss. Itshould be noted that the placement of the magnetic field 94 relative tothe separator member 70 is illustrative only. The magnetic field 94could be placed closer to or further away from the separator member 70.For instance, if placed closer, the magnetic field 94 would magnetizethe magnetic particles before the free jet 86 breaks up into therotating elements 88. If placed further away, the magnetic field 94would magnetize the magnetic particles after the rotating elements 88have hardened.

If high viscosity hardenable liquids are used, as disclosed in U.S.patent application Ser. No. 08/716,672 by Sheridon et al., then the freejet 86 forms filaments which are suitable for making cylindricallysymmetric rotating elements 34. As shown in FIG. 5, with respect tospheres 88, if the filaments are passed between a magnetic field 94while they are being spun, the magnetic pigment will be magnetized andall filaments will be identically magnetized.

FIG. 6 shows an implementation of the technique described above withrespect to FIG. 5 using a spinning disk assembly 96. Like elements aregiven the same reference numerals as used in FIG. 5. The separatormember 70 is implemented by a spinning disk which rotates around aspindle 98, The separator member has two surfaces 72, 74 connected atedge 76, over which two fine planar streams 80, 82 of low viscosityhardenable material are flowing. In this example, planar stream 80contains a white pigment while planar stream 82 contains a magneticpigment similar to that used in the manufacture of magnetic tapes, suchas black magnetic pigment type 031182 by Wright Industries, Brooklyn,N.Y. either alone or in conjunction with other black pigments as areknown in the art. The planar streams 80, 82 form an outboard reservoir84 of liquid which contains equal, side-by side, amounts of each liquidfrom each planar stream 80, 82.

A free jet 86 of liquid is formed from the reservoir 84, in anapproximately planar area outward from the reservoir, when the flow rateof the liquids away from the edge 76 is great enough. The free jet 86breaks up into rotating elements 88 at its distal end. While therotating elements 88 are in flight from the free jet 86, they passthrough a steady magnetic field 94, which is shown being created by twotorous-shaped magnets 90, 92. As the rotating elements 88 pass throughthe magnetic field the section of the rotating elements 88 containingthe magnetic pigment will become magnetized. As the rotating elements 88are identically oriented with respect to their trajectories, they willbe identically magnetized with respect to their geometric poles.

If high viscosity hardenable liquids are used, then identicallymagnetized filaments, suitable for cylindrically symmetric rotatingelements will be created instead of spheres 88.

Sheet Fabrication Method 1

The gyricon sheet 46 with the soft magnetic material pad 42 can befabricated by first mixing the magnetized rotating element 34 with asoft magnetic material powder such as Black Pigment #V-302 by the FerroCorp, Brooklyn N.Y. The soft magnetic material particles 100 wouldcluster around the magnetized segment 40 as shown in FIG. 7. Surplusparticles 100 are removed from the rotating elements 34 by placing themin a fluidized bed or placing them on a screen 102 where they are washedwith controlled air jets 104 as shown in FIG. 8. The rotating elements34 are then mixed with a liquid resin and spread out onto a thin layeron a flat surface to form an uncured sheet 106 a shown in FIG. 9. Auniform magnetic field 108 is applied to cause the magnetized rotatingelements 34 to rotate into common alignment with each other. When thishappens the magnetic pigment 100 will also migrate to remain adjacent tothe magnetized segment 40. After alignment has occurred, as shown inFIG. 9, but before removing the magnetic field 108, the sheet is curedinto a tough silicone elastomer, as is known in the art. After curing,the elastomer is swelled by placing it into an oil bath as is also knowin the art. The powdered soft magnetic material particles 100 have thusbeen incorporated into the elastomer matrix to form the soft magneticmaterial pad 42 in the vicinity of the magnetized segment 40 of therotating element 34 shown in FIG. 4.

It will be understood that the shape of the soft magnetic material pad42 will tend to conform to the shape of the rotating element 34 due tothe method of manufacture of the soft magnetic material pad 42. Forinstance, the soft magnetic material pad 42 may tend to curve slightlyand mimic the shape of the rotating element 34. Furthermore it will beunderstood that FIG. 4 shows a cross-sectional view for either aspherically or cylindrically shaped rotating element 34 the pad willtend to form in a circular shape for a spherically shaped rotatingelement 34 or in an elongated shape for a cylindrically shaped rotatingelement.

Sheet Fabrication Method 2

Alternatively, the gyricon sheet 46 with the soft magnetic material pad42 can be fabricated as part of an addition to a gyricon sheet which hasbeen made using any method of creating a gyricon sheet including thosedescribed hereinabove or any of the references incorporated hereinaboveand using magnetizable elements. The sheet may be constructed usingeither rotating elements 34 that are pre-magnetized, as used above, orcontaining magnetizable but not yet magnetized rotating elements 34. Ifthe sheet 46 is constructed using rotating elements 34 that have notbeen magnetized, then once the sheet has been constructed and soaked inoil so that the rotating elements 34 may rotate, a uniform electricfield is applied to orient the rotating elements 34 in a commondirection, as is known in the art. Once the rotating elements 34 havebeen oriented in a common direction a strong magnetic field 94, asdetailed hereinbefore, is applied to magnetize the rotating elements 34uniformly as shown in FIG. 10.

In either case, once a sheet 46 has been obtained with uniformlymagnetized rotatable particles 34 oriented in a common direction, a thinlayer 110 of uncured or molten material, such as an uncured elastomer,epoxy or a molten polymer, containing powdered soft magnetic materialparticles 100 is adhered to one side of the gyricon sheet. The powderedsoft magnetic material particles 100 will be attracted towards themagnetic segments 40 of the rotating elements and migrate to form thesoft magnetic material pads 42 as shown in FIG. 11. At this point thethin layer 110 containing the particles 100 is cured or otherwisesolidified, locking the soft magnetic material pads 42 in place.

It will be understood that the shape of the soft magnetic material pad42 will tend to conform to the shape of the rotating element 34 due tothe method of manufacture of the soft magnetic material pad 42. Forinstance, the soft magnetic material pad 42 may tend to curve slightlyand mimic the shape of the rotating element 34. Furthermore it will beunderstood that FIG. 4 shows a cross-sectional view for either aspherically or cylindrically shaped rotating element 34 and the pad 42will tend to form in a circular shape for a spherically shaped rotatingelement 34 or in an elongated shape for a cylindrically shaped rotatingelement.

Sheet Fabrication Method 3

Alternatively, the gyricon sheet 46 with the soft magnetic material pad42 can be formed by mixing into an uncured elastomer soft magneticmaterial powder 100 and magnetized rotating elements 34. This is formedinto an uncured sheet 106 on a surface, and the curing is delayed toallow the pigment particles 100 to be attracted to the magnetizedsegments 40 of the rotating elements 34. This pigment particles 100 willbe attracted to the magnetized segments 40 because the magnetizedsegments 40 create a very non-uniform magnetic field in theirvicinities. This field provides the mechanical force to move the pigmentparticles to the surface of the magnetized segments 40 of the rotatingelements. When this process is sufficiently complete, a uniform magneticfield 108, shown in this example being created using two magnets 89, 91is applied to the sheet, causing the rotating elements 34 and theattached soft magnetic material pigment particles 100, to rotate intocommon alignment. As is shown in FIG. 12. While this field iscontinuously applied the elastomer sheet is cured as is known in theart. The sheet can then be swelled, as is also known in the art.

It will be understood that the shape of the soft magnetic material pad42 will tend to conform to the shape of the rotating element 34 due tothe method of manufacture of the soft magnetic material pad 42. Forinstance, the soft magnetic material pad 42 may tend to curve slightlyand mimic the shape of the rotating element 34. Furthermore it will beunderstood that FIG. 4 shows a cross-sectional view for either aspherically or cylindrically shaped rotating element 34 the pad willtend to form in a circular shape for a spherically shaped rotatingelement 34 or in an elongated shape for a cylindrically shaped rotatingelement.

Gyricon Elements With Dual Latched States

As discussed hereinbefore, with respect to FIG. 4, a controlledthreshold is provided by using a rotating element which incorporates asingle magnetic segment interacting with a single soft magnetic materialpad located adjacent to the oil-filled cavity containing the rotatingelement. However, this provides threshold control for only therotational transition when the magnetized portion of the rotatingelement is adjacent to the soft magnetic material pad and the rotatingelement is being rotated so that this portion is at the opposite side ofthe cavity. This is good enough for many applications. For some passiveaddressing applications however, it is desired to rotate elements intoboth polarities electronically, without first erasing the whole image.In these applications there is a need for two thresholds, one for eachrotation state.

Configuration 1--Element With A Single Magnetic Portion And Two Pads

In FIG. 13 is shown a cross-section of a portion of a magneticallyassisted Gyricon sheet 46. This is the same sheet as shown in FIG. 4,with a slight modification, and therefore the same reference numeralswill be used for the same elements. A black and white bichromal rotatingelement 34 is shown in which the black magnetized segment 40 is madefrom black pigments, some of which are permanently magnetizable. Itshould be noted that black and white are used here for illustrativepurposes only and any colors could be chosen. The white unmagnetizedsegment 38 is constructed from the usual materials and is notmagnetizable. This rotating element 34 is contained in an oil filledcavity 36. In contrast to FIG. 4, two soft magnetic material pads 42 areincorporated near the cavity structure of each rotating element in anopposed configuration. The rotating element 34 is also made frommaterials that develop electrical potentials in contact with the liquidin the oil filled cavity 36 and in the presence of the electrical field,so that the segments 38, 40 of the rotating element 34 develop differentelectrical potentials from each other.

When the black magnetized segment 40 of the rotating element 34 isadjacent to either of the soft magnetic material pads 42 embedded nextto the oil filled cavity 36, a strong magnetic force tends to hold therotating element 34 in place. This is because the distance between themagnetized portion of the rotating element 34 and the soft magneticmaterial pad 42 is very short compared to the dimensions of themagnetized portion of the rotating element, thus providing a strongmagnetic field. For practical purposes, the separation distance betweenD_(s) the rotating element 34 and the soft magnetic material pad 42should be no more than the diameter d of the rotating element 34multiplied by a factor of three. The soft magnetic material pad 42should preferably have a length l no smaller than 1/4 of the rotatingelement diameter d. The only restriction on the upper limit of thelength l is that it must not be so large as to interfere withsurrounding rotational elements or their soft magnetic material pads.This will be dictated by the packing density of the gyricon sheet 46.Depending on the packing density, and the length l of the soft magneticmaterial pad 42 can be as large as the diameter d of the rotatingelement 34 or even twice as large as the diameter d of the rotatingelement 34 or more. This magnetic force will cause the rotating element34 to be attached to the oil filled cavity 36 wall, and will alsorequire a larger electrical field than otherwise to cause the rotatingelement 34 to start to rotate in the oil filled cavity 36. Once therotating element 34 has rotated a short distance it will experience amuch reduced force from interaction with the soft magnetic material pad42 and the motion of the rotating element 34 will be dominated by theapplied electrical field. Therefore if a rotating element 34 is alignedin its oil filled cavity 36 in an orientation in which the magnetizedsegment 40 is adjacent to either of the soft magnetic material pads 42,a strong electrical field will be required to initiate rotation. Bycontrolling the density, for uniformity among particles, and the type ofmagnetic particles of the magnetized segment 40 of the rotating element34, the threshold value of electrical field required to initiaterotation can be made uniform and sharp. This is because the effects ofthe magnetic field on the threshold voltage will dominate over othereffects on the threshold voltage.

Using a rotating element 34 with a magnetic segment and two softmagnetic material pads provides thresholds for both states of rotation.Two magnetic soft magnetic material pads 42 are used, one for eachdesired orientation of the rotating element, and therefore, thethreshold is controlled for both states in contrast to the embodimentdescribed above and shown in FIG. 4. This enhancement would be useful inproviding for the sharp threshold and image storage requirements neededto effectively implement passive addressing.

Sheet Fabrication

This sheet may be fabricated using any of the methods described above toobtain the initial sheet. However, this results in providing only onesoft magnetic material pad 42, and two soft magnetic material pads 42are desired. Therefore, once an initial sheet is fabricated having onesoft magnetic material pad 42 the second pad can be provided using thethin layer technique described above and discussed with respect to FIG.14.

Once a plasticized sheet 46 has been obtained with uniformly magnetizedrotatable particles 34, an electric field can be applied as known in theart to orient the magnetized rotatable particles 34 in a commondirection where the magnetized segment 40 has been rotated away from thesoft magnetic material pad 42. Subsequently, a thin layer 110 of uncuredor molten material, such as an uncured elastomer, epoxy or a moltenpolymer, containing powdered soft magnetic material particles 100 isadhered to the side of the gyricon sheet which does not have softferromagnetic materials pads 42 and towards which the magnetizedsegments 40 of the rotatable elements 34 have been oriented. Thepowdered soft magnetic material particles 100 will be attracted towardsthe magnetic segments 40 of the rotating elements 34 and form the softmagnetic material pads 42 as shown in FIG. 14. At this point the thinlayer 110 containing the particles 100 is cured or otherwise solidified,locking the soft magnetic material pads 42 in place.

It will be understood that the shape of the soft magnetic material pad42 will tend to conform to the shape of the rotating element 34 due tothe method of manufacture of the soft magnetic material pad 42. Forinstance, the soft magnetic material pad 42 may tend to curve slightlyand mimic the shape of the rotating element 34. Furthermore it will beunderstood that FIG. 4 shows a cross-sectional view for either aspherically or cylindrically shaped rotating element 34 the pad willtend to form in a circular shape for a spherically shaped rotatingelement 34 or in an elongated shape for a cylindrically shaped rotatingelement.

Configuration 2--Element With Dual Magnetic Portions And A Single Pad

FIG. 15 shows a cross-section of a gyricon sheet 46. Again, this sheetis a variant of the sheet 46 shown in FIG. 4 and the same referencenumerals will be used for the same elements. The sheet 46 is made from atransparent optical medium 44 with an oil filled cavity 36 whichcontains a rotating element 52. Rotating element 52 is a spherically orcylindrically symmetric bichromal element containing, for example, ablack segment 54 and a white segment 56. Additionally, rotating element52 contains two small polar magnetic segments 58, 60 where polarmagnetic segment 58 is located adjacent to the black segment 54 andpolar magnetic segment 60 is located adjacent to the white segment 56.Further, if the junction J of the segments 54, 56 is viewed as anequatorial line then the magnetic segments are located at the "poles" ofthe rotating element 52.

A single soft magnetic material pad 42 is contained within thetransparent optical medium 44 adjacent to the oil filled cavity 36, asshown in the FIG. 15. The two magnetic segments 58, 60 interact with thesoft magnetic material pad 42 to provide the rotating element with tworotational positions in which the threshold is controlled by magneticfields. Each magnetic segment 58, 60 interacts with the soft magneticmaterial pad 42 in the same manner as described above with respect toFIG. 4. That is, when one of the magnetic segments 58, 60 of therotating element 52 is adjacent to the soft magnetic material pad 42that is embedded next to the oil filled cavity 36, a strong magneticforce tends to hold the rotating element 52 in place. This is becausethe distance between the magnetic segments 58, 60 of the rotatingelement 52 and the soft magnetic material pad 42 is very short comparedto the dimensions of the magnetized portions of the rotating element.For practical purposes, the separation distance D_(s) between therotating element 34 and the soft magnetic material pad 42 should be nomore than the diameter d of the rotating element 34 multiplied by afactor of three. The soft magnetic material pad 42 should preferablyhave a length l no smaller than 1/4 of the rotating element diameter d.The only restriction on the upper limit of the length l is that it mustnot be so large as to interfere with surrounding rotational elements ortheir soft magnetic material pads. This will be dictated by the packingdensity of the gyricon sheet 46. Depending on the packing density, thelength l of the soft magnetic material pad 42 can be as large as thediameter d of the rotating element 34 or even twice as large as thediameter d of the rotating element 34 or more. This magnetic force willcause the rotating element 52 to be latched in place, and will alsorequire a larger electrical field than otherwise to cause the rotatingelement 52 to start to rotate in the oil filled cavity 62. Once therotating element 52 has rotated a short distance the polar magneticsegment will experience a much reduced force from the soft magneticmaterial pad 42 it had been adjacent to and the motion of the rotatingelement 52 will be dominated by the applied electrical field.

Using a rotating element 52 with two polar magnetic segments 58, 60 andone soft magnetic material pad 42 provides thresholds for both states ofrotation. Two polar magnetic segments 58, 60 are used, one for eachdesired orientation of the rotating element 52, and therefore, thethreshold is controlled for both states, in contrast to the embodimentdescribed above and discussed with respect to in FIG. 4. Additionally,the magnetized portion of the rotating element 52 is confined to twosmall polar magnetic segments 58, 60. While it would be possible toimplement this variation using a rotating element which uses the largemagnetized segment 40 of the rotating element shown in FIG. 4 combinedwith a single polar magnetic segment 58, 60 of the type shown in therotating element shown in FIG. 18, using two small polar magneticsegments 58, 60 as shown in FIG. 18 provides a finer control, moreprecise control. These enhancements would be useful in providing for thesharp threshold and image storage requirements needed to effectivelyimplement passive addressing.

Rotating Element Fabrication--Polar Magnetic Segment

This rotating element can be fabricated as know in the art with amodified multiple rotating disk assembly, as described in U.S. Pat. No.5,717,514 by Sheridon titled "Polychromal Segmented Balls For A TwistingBall Display", and U.S. patent application Ser. No. 08/960,865 bySheridon et al. titled "Twisting Cylinder Display" and incorporated byreference hereinabove, or other planar stream/free jet type devices suchas those disclosed in U.S. Pat. No. 5,344,594, titled "Method ForFabrication Of Multicolored Balls For A Twisting Ball Display", bySheridon and also incorporated by reference hereinabove. Themanufacturing devices discussed below are variations on those discussedwith respect to FIGS. 5 and 6 and the same reference numerals will beused to identify the same elements.

To generally understand the concepts of magnetic rotating elementgeneration, FIG. 16 shows two separator members 70, each having twoopposed surfaces 72, 74 connected at edge 76, over each of which twofine planar streams 80, 82 of hardenable material are flowing. In thisexample, outward planar streams 80 contain a magnetic pigment similar tothat used in the manufacture of magnetic tapes, such as black magneticpigment Type 031182 by Wright Industries, Brooklyn, N.Y. either alone orin conjunction with other black pigments as are known in the art, whileinward planar streams 82 contain each contain one of the pigments usedto provide the segment colorations of the rotating elements 88. Forinstance one of the inward streams 82 may contain a white pigment whilethe other of inward planar streams 82 may contain a black pigment. Theplanar streams 80, 82 combine to form a free jet 86 of liquid whichcontains side-by side, amounts of each liquid from each planar stream80, 82 from each separator member 76. In order to create the smaller,polar magnetic segments the outward planar streams 80 may contain asmaller volume of material than the inward planar streams 82.

The free jet 86 of liquid is formed when the flow rate of the liquidsaway from the edge 76 is great enough. Methods known in the art forcreating a free jet 86 include a spinning disk assembly as described inU.S. Pat. No. 5,717,514 by Sheridon titled "Polychromal Segmented BallsFor A Twisting Ball Display", and a jet assembly, a planar sheet liquidsheet, and a cylindrical liquid sheet described in U.S. Pat. No.5,344,594, titled "Method For Fabrication Of Multicolored Balls For ATwisting Ball Display", by Sheridon, any of which may be suitably used.If low viscosity hardenable liquids are used the free jet 86 breaks upinto rotating elements 88 at its distal end as shown in FIG. 16.

While the rotating elements 88 are in flight from the free jet 86, theypass through a steady magnetic field 94, which is shown being created bytwo magnets 90, 92. As the rotating elements 88 pass through themagnetic field the section of the rotating elements 88 containing themagnetic pigment will become magnetized, As the rotating elements 88 areidentically oriented with respect to their trajectories, they will beidentically magnetized with respect to their geometric poles. The steadymagnetic field may be created by any number of ways known in the art,for example, a permanent magnet, an electric field or a direct currentthrough a coil. To properly magnetize the magnetic pigment the magneticfield 94 should be at least 50 gauss.

If high viscosity hardenable liquids are used, as disclosed in U.S.patent application Ser. No. 08/716,672 by Sheridon et al. then the freejet 86 forms filaments which are suitable for making cylindricallysymmetric rotating elements 34. As shown in FIG. 16, with respect tospheres 88, if the filaments are passed between a magnetic field 94while they are being spun, the magnetic pigment will be magnetized andall filaments will be identically magnetized.

FIG. 17 shows an implementation of the technique described above withrespect to FIG. 16 using a multiple spinning disk assembly 96. Likeelements are given the same reference numerals as used in FIG. 16. Thetwo separator members 70 are each implemented by a spinning disk whichrotates around a single spindle 98. Each separator member has twosurfaces 72, 74 connected at edge 76. Over each separator member 70 twofine planar streams 80, 82 of low viscosity hardenable material areflowing. In this example, outward planar streams 80 contain a magneticpigment similar to that used in the manufacture of magnetic tapes, suchas black magnetic pigment Type 031182 by Wright Industries, Brooklyn,N.Y. either alone or in conjunction with other black pigments as areknown in the art, while inward planar streams 82 contain each containone of the pigments used to provide the segmental colorations of therotating elements 88. For instance one of the inward streams 82 maycontain a white pigment while the other of inward planar streams 82 maycontain a black pigment. The planar streams 80,82 combine to form a freejet 86 of liquid which contains side-by side, amounts of each liquidfrom each planar stream 80, 82 from each separator member 76. In orderto create the smaller polar magnetic segments the outward planar streams80 may contain a smaller volume of material than the inward planarstreams 82.

A free jet 86 of liquid is formed from the reservoir 84 when the flowrate of the liquids away from the edge 76 is great enough. The free jet86 breaks up into rotating elements 88 at its distal end. While therotating elements 88 are in flight from the free jet 86, they passthrough a steady magnetic field 94, which is shown being created by twotorous-shaped magnets 90, 92. As the rotating elements 88 pass throughthe magnetic field the section of the rotating elements 88 containingthe magnetic pigment will become magnetized, As the rotating elements 88are identically oriented with respect to their trajectories, they willbe identically magnetized with respect to their geometric poles.

If high viscosity hardenable liquids are used, then identicallymagnetized filaments, suitable for cylindrically symmetric rotatingelements will be created instead of spheres 88.

In the event that a simpler rotating element 62, using a large blackmagnetized segment 64 and only one small polar magnetic segment 68separated by an white or colored unmagnetized segment 66, as shown inFIG. 17a, is desired then only three of the surfaces would be used. Onesurface would be used for the large black magnetized segment, one forthe white or colored unmagnetized segment and one for the magnetic polarsegment. This element could be used interchangeably with the abovedescribed element having two polar, magnetic segment.

Furthermore, this apparatus can also be used to create a rotatingelement 320 with only a single small, polar magnetic segment 326 and twounmagnetized segments 322, 324 as shown in FIG. 17b. Again, only threesurfaces would be used. One surface would be used for the black segment,one for the white segment and one for the polar magnetic segment. Suchan element could be used interchangeably with the segmentally chargedrotating element described above with respect to FIG. 4.

Sheet Fabrication Method

The sheet 46 can be fabricated using the thin layer technique asdescribed above and shown in FIG. 11. Once a plasticized sheet 46 hasbeen obtained with uniformly magnetized rotatable particles 34, anelectric field can be applied as known in the art to orient themagnetized rotatable particles 34 in a common direction. Subsequently, athin layer 110 of uncured or molten material, such as an uncuredelastomer, epoxy or a molten polymer, containing powdered soft magneticmaterial particles 100 is adhered to one side of the gyricon. Thepowdered soft magnetic material particles 100 will be attracted towardsthe polar magnetic segment 58 of the rotating elements and form the softmagnetic material pads 42 as described earlier. At this point the thinlayer 110 containing the particles 100 is cured or otherwise solidified,locking the soft magnetic material pads 42 in place.

It will be understood that the shape of the soft magnetic material pad42 will tend to conform to the shape of the rotating element 34 due tothe method of manufacture of the soft magnetic material pad 42. Forinstance, the soft magnetic material pad 42 may tend to curve slightlyand mimic the shape of the rotating element 34. Furthermore it will beunderstood that FIG. 4 shows a cross-sectional view for either aspherically or cylindrically shaped rotating element 34 the pad willtend to form in a circular shape for a spherically shaped rotatingelement 34 or in an elongated shape for a cylindrically shaped rotatingelement.

Configuration 3--Element With Dual Magnetic Portions And Two Pads

FIG. 18 shows a cross-section of a gyricon sheet 46. Again, this sheetis a variant of the sheet 46 shown in FIG. 4 and the same referencenumerals will be used for the same elements. The sheet 46 is made from atransparent optical medium 44 with an oil filled cavity 36 whichcontains a rotating element 52. Rotating element 52 is a bichromalelement containing, for example, a black segment 54 and a white segment56. Additionally, rotating element 52 contains two small polar magneticsegments 58, 60 where polar magnetic segment 58 is located adjacent tothe black segment 54 and polar magnetic segment 60 is located adjacentto the white segment 56. Further, if the junction 70 of the segments 54,56 is viewed as an equatorial line then the magnetic segments 58, 60 arelocated at the "poles" of the rotating element 52.

Two soft magnetic material pads 42 are contained within the transparentoptical medium 64 adjacent to the oil filled cavity 36 in an opposedconfiguration, as shown in the FIG. 18. The two magnetic segments 58, 60interact with the two soft magnetic material pads 42 to provide therotating element with two rotational positions in which the threshold iscontrolled by magnetic fields. Each magnetic segment 58, 60 interactswith one of the two soft magnetic material pads 42 in the same manner asdescribed above with respect to FIG. 4. That is, when one of themagnetic segments 58, 60 of the rotating element 52 is adjacent to oneof the soft magnetic material pads 42 that is embedded next to the oilfilled cavity 36, a strong magnetic force tends to hold the rotatingelement 52 in place. This is because the distance between the magneticsegments 58, 60 of the rotating element 52 and the soft magneticmaterial pad 42 is very short compared to the dimensions of themagnetized portions of the rotating element. For practical purposes, theseparation distance D_(s) between the rotating element 52 and the softmagnetic material pad 42 should be no more than the diameter d of therotating element 52 multiplied by a factor of three. The soft magneticmaterial pad 42 should preferably have a length l no smaller than 1/4 ofthe rotating element diameter d. The only restriction on the upper limitof the length l is that it must not be so large as to interfere withsurrounding rotational elements or their soft magnetic material pads.This will be dictated by the packing density of the gyricon sheet 46.Depending on the packing density, the length l of the soft magneticmaterial pad 42 can be as large as the diameter d of the rotatingelement 34 or even twice as large as the diameter d of the rotatingelement 34 or more. This magnetic force will cause the rotating element52 to be latched in place, and will also require a larger electricalfield than otherwise to cause the rotating element 52 to start to rotatein the oil filled cavity 62. Once the rotating element 52 has rotated ashort distance it will experience a much reduced force from the softmagnetic material pad 42 it had been adjacent to and the motion of therotating element 52 will be dominated by the applied electrical field.

Using a rotating element 52 with two magnetic segments and two softmagnetic material pads provides thresholds for both states of rotation.Two magnetic segments and two soft magnetic material pads are used, onefor each desired orientation of the rotating element, and therefore, thethreshold is controlled for both states, in contrast to the embodimentdescribed above and shown in FIG. 4. Additionally, the magnetizedportion of the rotating element is confined to two small polar magneticsegments. While it would be possible to implement this variation using arotating element which uses the magnetized segment 40 of the rotatingelement shown in FIG. 4 combined with a single polar magnetic segment 58of the type shown in the rotating element shown in FIG. 18, using twosmall polar magnetic segments 58, 60 as shown in FIG. 18 provides afiner more precise control. These enhancements would be useful inproviding for the sharp threshold and image storage requirements neededto effectively implement passive addressing.

Methods of fabricating the rotating elements 32 and the sheets utilizingthose elements have been detailed above with respect to otherconfigurations and are also applicable in this embodiment.

Configuration 4--Soft Magnetic Material Rings

The various embodiments discussed above placed the soft magneticmaterial pads near one or both poles of a spherical rotating element.While this is a convenient location for the soft ferromagnetic material,it suffers from the disadvantage that light can not travel through thesoft magnetic material and therefore some of the desirable visualcharacteristics of the gyricon sheet may be disrupted. For instance, thebrightness of the display may be lessened. Therefore, it would bedesirable to fabricate a gyricon device using magnetic fields and softmagnetic material where the soft magnetic material is not contained inthe optical viewing path. The following description describes anothervariant of the sheet shown in FIG. 4 and uses the same referencenumerals for like elements.

FIG. 19 shows the gyricon sheet 46 made from transparent optical medium44 with an oil filled cavity 36 as before. However, the oil filledcavity 36 contains a bichromal rotating element 112 with an end segment114 of a first color and an end segment 116 of a second color differentfrom the first color. Interposed between the end segments 114, 116 is amagnetic segment 118. The magnetic segment 118 is a relatively thinsegment, of thickness t, approximately slicing through the center of therotating element 112. Instead of discreet soft magnetic material padsthere is now a soft magnetic material pad in the shape of a softmagnetic material loop or ring 120 surrounding the oil filled cavity 36again, at approximately the centerline of the oil filled cavity 36. Itshould be noted that if the rotating element 112 is sphericallysymmetric than the soft magnetic material ring 120 will be essentiallyround, as for instance, the rings around Saturn. However, if therotating element 112 is cylindrically symmetric then the soft magneticmaterial ring 120 will be an elongated shape. The rotating element 112has two equally stable states of orientation, each with a sharpthreshold mostly controlled by magnetic fields between the magneticsegment 118 and the soft magnetic material ring 120. For practicalpurposes, the separation distance D_(s) between the rotating element 112and the soft magnetic material ring 120 should be no more than thethickness t of the magnetic segment 118 multiplied by a factor of four.The soft magnetic material ring 120 should preferably have a length l nosmaller than 1/4 of the magnetic segment thickness t. The onlyrestriction on the upper limit of the length l is that it must not be solarge as to interfere with surrounding rotational elements or their softmagnetic material pads. This will be dictated by the packing density ofthe gyricon sheet 46. Depending on the packing density, the length l ofthe soft magnetic material ring 120 can be as large as the thickness tof the magnetic segment 118 or even four as large as the thickness t ofthe magnetic segment 118 or more.

Operation is similar to the examples described above. The magneticsegment 118, interacts with the soft magnetic material ring 120 in thesame manner as described above with respect to FIGS. 4 and 18. That is,when the magnetic segment 118 of the rotating element 112 is adjacent tothe soft magnetic material ring 120 embedded next to the oil filledcavity 36, a strong magnetic force tends to hold the rotating element112 in place. This magnetic force will cause the rotating element 112 tobe latched in place, and will also require a larger electrical fieldthan otherwise to cause the rotating element 112 to start to rotate inthe oil filled cavity 36. Once the rotating element 112 has rotated ashort distance it will experience a much reduced force from the softmagnetic material ring 120 and the motion of the rotating element 112will be dominated by the applied electrical field.

This configuration allows for latching with either side of the rotatingelement 112 to be viewable and unobstructed by magnetic latchingelements.

Rotating Element Fabrication

Fabrication of the rotating element 112 shown in FIG. 19, again requiresmodified multiple rotating disk assembly, as described in U.S. Pat. No.5,717,514 by Sheridon titled "Polychromal Segmented Balls For A TwistingBall Display", and U.S. patent application Ser. No. 08/960,865 bySheridon et al. titled "Twisting Cylinder Display" and incorporated byreference hereinabove, or other planar stream/free jet type devices suchas those disclosed in U.S. Pat. No. 5,344,594, titled "Method ForFabrication Of Multicolored Balls For A Twisting Ball Display", bySheridon and also incorporated by reference hereinabove as discussedwith reference to creating the rotating elements that have one or moresmall, pole magnetic segments.

Two separator members would be required but only three of the liquiddelivery surfaces would be used. One surface would be used for one ofthe colored end segments, one surface would be used for the other,differently colored end segment and one for the magnetic segment. Again,the magnetic segment in the individual rotating elements can bemagnetized by causing the rotating elements to pass through a magneticfield during the fabrication process, as illustrated in FIGS. 16 and 17.

Sheet Fabrication

The soft magnetic material ring 120 can also be fabricated by utilizingthe same process as the Sheet Fabrication Method 1 described above forthe soft magnetic material pads with a single latched state as shown inFIG. 9. The magnetized rotating element 112 is mixed with a softmagnetic material powder such as Black Pigment #V-302 by the FerroCorporation, Cleveland, Ohio. The powdered particles would clusteraround the magnetized magnetic segment. Again, surplus particles can beremoved by the use of a fluidized bed or by placing the balls on ascreen and washing them with an air stream. The rotating element 112 isthen mixed with a liquid elastomer and spread out into a thin layer on aflat surface to form a sheet. This sheet is next placed between two flatmagnets and the magnetic field created by these magnets will cause themagnetized rotating elements to rotate into a common alignment with eachother. With this magnetic field present the sheet is cured into a toughsilicone elastomer, as is known in the art. After curing, the elastomeris swelled by placing it into an oil bath as is also know in the art.The powdered soft magnetic material particles have thus beenincorporated into the elastomer matrix, to form the soft magneticmaterial ring 120 in the vicinity of the magnetic segment 118 of therotating element 112.

Alternatively, the soft magnetic material ring 120 can also befabricated by utilizing the same process as the Sheet Fabrication Method3 described above for the soft magnetic material pads with a singlelatched state and shown in FIG. 12. The soft magnetic material ring 120can be formed by mixing into an uncured elastomer soft magnetic materialpowder and magnetized rotating elements 112. This is formed into anuncured sheet on a surface, and the curing is delayed to allow thepigment particles to be attracted to the magnetized segments of therotating elements 112. The pigment particles will be attracted to themagnetized segments because the magnetized segments create a verynon-uniform magnetic field in their vicinities. This field provides themechanical force to move the pigment particles to the surface of themagnetized segments of the rotating elements. When this process issufficiently complete, a uniform magnetic field is applied to the sheet,causing the rotating elements and the attached soft magnetic materialpigment particles, to rotate into common alignment. While this field iscontinuously applied the elastomer sheet is cured as is known in theart. The sheet can then be swelled, as is also known in the art.

Hard Magnetic Material Trap Devices

Up to this point, the description has been focussed on describinggyricon devices which utilize soft magnetic materials in conjunctionwith magnetized elements. That is material capable of developing astrong magnetic dipole strength while exposed to a strong externalmagnetic field, but that is not capable of retaining significant remnantmagnetism when no longer exposed to the external field. However, formsare also possible which utilize hard magnetic materials, that ismaterial capable of retaining significant magnetism without the aid ofan external field. These devices are described as magnetic trap devicesbecause the continuous magnetic field serves to trap the rotatingelement in its place until an extra strong electric field or an oppositepolarity magnetic field is applied to overcome the magnetic field andallow the rotation of the element.

Gyricon With Magnetic Pads

The rotatable optical Gyricon elements described above in FIGS. 1, 2,and 3 are constrained to stay within cavities that emulate the shapes ofthe elements. Therefore it is possible to create magnetic traps thatensure the bistability of switching between optical states, and thatcreate modifiable switching thresholds. FIG. 20 shows a modification ofthe structure shown in FIG. 4, and therefore the same reference numeralswill be used for the same elements.

FIG. 20 shows a gyricon sheet 46 made from a transparent optical medium44 with an oil filled cavity 36 enclosing a rotating element 34 whereone segment 40 is one color and the other segment 38 is a second color.The rotating element 34 shown in FIG. 20 is the same bichromal rotatingelement 34 as shown in FIG. 4 and the segment 40 of the rotating element34 is made from pigments, at least some of which are permanentlymagnetizable. As noted with regard to FIG. 17b, rotating element 320 maybe used interchangeably with the rotating element 34 shown in thisfigure and other figures throughout. Adjacent to each oil filled cavity36 and localized to the dimensions of the rotating element 34 is a pad134 of permanently magnetized particles 132. This is different from thestructure shown in FIG. 4 which had a pad made of soft magneticmaterial. The pad 134 and the magnetized segment 40 of the rotatingelement 34 are magnetized in such a way that when the magnetized segment40 of the rotating element 34 is rotated to be adjacent to the pad 134the magnetized segment 40 and the pad 134 are maximally attracted to oneanother. This can be accomplished by polarizing the magnetic segment 40and the pad 132 as shown in the diagram where "N" and "S" represent thenorth and south poles respectively. Thus, when the rotating elements arerotated as shown in FIG. 20 they are held in place by a magnetic fieldH. In addition, since the magnetic pads 134 are of dimensions comparableto those of the magnetic segment 40, the magnetic field H created bythem in the vicinity of the rotating element 34 is strongly non-uniform.

FIG. 21 shows the same gyricon sheet 46 with the rotating element 34rotated in the opposite configuration as shown in FIG. 20. That is, therotating element 34 has been rotated such that the magnetic segment 40is facing away from the pad 134. As can be seen in FIG. 21, thepolarization of the magnetic segment 40 and the pad 134 are such thatlike poles are facing each other and the magnetic segment 40 and the pad134 will now repel each other.

For the configuration shown in FIG. 21 to be stable, the rotatingelement 34 must not undergo slip rotation. Experimental observationconfirms that when rotating elements are switched from one optical stateto another they rotate as they cross the cavities. Sometimes they evenroll along the cavity walls. When they reach the cavity walls adjacentto the addressing electrode they stop all forms of rotary motion. Theserotating elements never undergo slip rotation in contact with cavitywalls, only rolling rotation.

Thus the rotating element 34 with its magnetic segment 40 facing upwardis pushed against the cavity wall by the magnetic field. It can rollalong the cavity wall, but once it reaches the highest portion of thecavity wall any further rotation will move the magnetic segment 40closer to the magnetic pad 134, a movement resisted by the repellingforce of the magnetic field H. Thus this is a second stable orientationof the rotating element with respect to magnetic field H.

This sheet can be fabricated using any of the methods previouslydescribed for fabricating a soft magnetic material pad device that is agyricon with a single latched state and substituting a permanentlymagnetizable particle, such as Black Magnetic Pigment type 031182 byWright Industries, Brooklyn, N.Y., for the soft magnetic materialparticles.

Gyricon With Etched Rubber Magnet

Other means of implementing the strongly non-uniform magnetic field Hwith dimensions comparable to those of the magnetic segment exist. FIG.22 shows an alternative implementation and the same reference numeralswill be used to identify the same elements. Again, the gyricon sheet 46is made from the transparent optical medium 44 with an oil filled cavity36 enclosing a rotating element 34 although rotating element 320 shownin FIG. 17b could also be used.

However, instead of the pad 134 of permanently magnetized particles 132a magnetic pad 152 has been adhered to one surface of the gyricon sheet.The magnetic pad 152 is constructed from a uniform permanentlymagnetized rubber sheet which has had etched areas 156 removed to createmagnetic hills 154. The etching depth determines the strength of thenon-uniform component of the magnetic field created by this magnet. Themagnetic hills 154 have dimensions of the same order as the rotatingelement 34 and the magnetic pad 152 is aligned with the gyricon sheet 46such that each magnetic hill 154 is aligned with an oil-filled cavity36. The magnetized segment 40 of the rotating element 34 is magnetizedin such a way that when the magnetized segment 40 of the rotatingelement 34 is rotated to be adjacent to the magnetic hill 154 of themagnetic pad 152, the magnetized segment 40 and the magnetic hill 154are maximally attracted to one another. This can be accomplished bypolarizing the magnetic segment 40 and the magnetic hill 154 as shown inthe diagram where "N" and "S" represent the north and south polesrespectively. Thus, when the rotating elements are rotated as shown inFIG. 22 they are held in place by a magnetic field H. In addition, sincethe magnetic hills 154 are of dimensions comparable to those of themagnetic segment 150, the magnetic field H created by them is stronglynon-uniform.

The gyricon sheet 46 can be made using any of the previously knowntechniques for creating gyricon sheets and utilizing magnetizablerotating elements, but in particular manufacturing techniques whichproduce a regular array of rotating elements within a sheet willsimplify production and alignment of the magnetic pad 152. One suchmanufacturing technique is the "eggcrate" display disclosed in U.S. Pat.No. 5,777,782 by Sheridon, titled "Auxiliary Optics For A Twisting BallDisplay" and herein incorporated by reference. The "eggcrate" displayproduces a highly, regular geometric pattern of rotating elements whichallows for the tight registration and alignment of rotating elementswith auxiliary components such as optical components or in this case apatterned magnetic pad.

Magnetic Pad Fabrication Method 1

The patterned magnetic pad 152 can be made by taking a sheet of"rubberized magnet", so called because it consists of a highconcentration of magnetic pigment particles dispersed in a rubberbinder, and patterning it by several known methods. One method is tocoat the surface of the rubberized magnet with a photoresist. Thephotoresist can then be masked and patterned as is known in the art. Ifa positive photoresist is used, the gyricon sheet 46 itself can be usedas the mask. The rotating elements 34 will block the light rays where amagnetic hill is desired. Using the gyricon sheet 46 as the mask toproduce the microstructure on the magnetic pad 152 insures that themagnetic hills 154 will be of the correct dimensional size and willcorrectly align with the rotating elements 34 thus relieving some of thealignment issues. Once the photoresist has been exposed and developedthe rubberized magnet can be etched using acids, such as nitric acid orsulfuric acid, or by using a plasma discharge etching process. The depthof the etching process, and thus the strength of the spatially varyingportion of the magnetic field, is determined by the strength of the acidand the amount of time spent in the acid.

Magnetic Pad Fabrication Method 2

In another known method, a thin aluminum mask can be created on thesheet of rubberized magnet. This sheet would be overcoated with aphotoresist, the latter being exposed using the Gyricon sheet 46 as aphoto-mask, as before. If a positive photoresist was used, the exposedareas will be removed. Etching with an acid, such as nitric acid, willleave optically reflective aluminum mirrors over regions correspondingto the rotating element 34 positions. The sheet of rubberized magnet cannow be exposed to a strong light source, such as from a laser or astrong incandescent lamp. The strong light source will destroy themagnetic properties of the rubberized magnet, where it is not protectedby the aluminum mask (which reflects the light), by heating the sheetabove the Curie point. Areas of the rubberized magnet heated above theirCurie point will lose their magnetism. Although this does not result inthe actual removal of material to form the magnetic hills 154, theeffect is the same.

Once the patterned magnetic pad 152 is made it can be aligned with andadhered to the gyricon sheet using appropriate adhesives or mechanicalclamping devices. Another approach would be to coat the surface of thegyricon sheet with a layer of uncured silicone rubber, and apply it tothe rubberized magnet. The edges of the thus made composite sheet wouldnext be clamped and the silicone rubber cured. The silicone rubbersheets would adhere poorly, but the clamps at the edges of the sheetwould prevent delamination.

Gyricon With Captured Magnetic Rotating Elements

Another means of implementing the strongly non-uniform magnetic field His with magnets having dimensions comparable to those of the magneticsegment. One embodiment is shown in FIG. 23. As the structure shown inFIG. 23 is a variant of the structure shown in FIG. 4, the samereference numerals will be used to identify the same elements. FIG. 23shows an implementation of a gyricon sheet 46 made from a transparentoptical medium 44 with an oil filled cavity 36 enclosing a rotatingelement 34 where one segment 38 is one color and the other segment 40 isa second color. The rotating element 34 shown in FIG. 23 is the samebichromal rotating element 34 as shown in FIG. 4, however the rotatingelement 320 shown in FIG. 17b, could also be used. The segment 40 of thebichromal rotating element is made from pigments, at least some of whichare permanently magnetizable, such as those used in magnetic recordingtapes. Examples of such pigments include Black Magnetic Pigment type031182 by Wright Industries, Brooklyn, N.Y.

However, instead of the pad 134 of permanently magnetized particles 132,as shown in FIG. 22, a second oil filled cavity 172 enclosing aferro-magnetic element 174 is provided. It should be noted that while around ferro-magnetic element is illustrated, it is not necessary, and itmay even be preferable that the ferro-magnetic element 174 not be round.This is due the constraint that the ferro-magnetic element 174 shouldnot itself rotate during the subsequent life of the gyricon sheet 46.This can accomplished in several ways. The first of these is to allowthe ferro-magnetic element to, at least partially, stick to thetransparent optical medium. This affect might also be accomplished bymaking the ferro-magnetic element 174 in a shape that is not amenable torotation, such as one having protrusions or sharp edges which wouldimpede rotation. This affect can also be accomplished by using magneticballs that release poorly from the silicone elastomer when it is swollenin plasticizing oil. Lastly, a thin permanently magnetic layer 176 couldbe used to hold the ferro-magnetic element 174 in the correctorientation. Such a thin magnetic layer could comprise a thin sheetmagnet, a thin layer of magnetic particles or other means. The advantageto using ferro-magnetic elements 174 that are spherical, is that thesame process used to make the rotating elements 34 can be used givinggood control of dimensions and insuring that the dimensions of theferro-magnetic elements 174 are of the same order as the rotatingelements 34.

The ferro-magnetic element 174 is aligned within the gyricon sheet 46such that each ferro-magnetic element 174 is aligned with a rotatingelement 34. The ferro-magnetic element 174 could be comprised ofmagnetic pigments, or for stronger magnetic fields rare earth materials.The magnetized segment 40 of the rotating element 34 is magnetized insuch a way that when the magnetized segment 40 of the rotating element34 is rotated to be adjacent to the ferro-magnetic element 174 themagnetized segment 40 and the ferro-magnetic element 174 are maximallyattracted to one another. This can be accomplished by polarizing themagnetic segment 40 and the ferro-magnetic element 174 as shown in thediagram where "N" and "S" represent the north and south polesrespectively. Thus, when the rotating elements are rotated as shown inFIG. 24 they are held in place by a magnetic field H. In addition, sincethe ferro-magnetic elements 174 are of dimensions comparable to those ofthe magnetic segment 40, the magnetic field H created by them isstrongly non-uniform.

Sheet Fabrication Method

The gyricon sheet 46 shown in FIG. 23 can be constructed by first makinga ferro-magnetic particle layer 178 comprising unmagnetizedferro-magnetic elements 34 in uncured elastomer 182 as shown in FIG. 24.The ferro-magnetic particle layer 178 should be made on a release layer180 such as Teflon. After the ferro-magnetic particle layer 178 has beenpartially cured a second layer of uncured elastomer 186 containingrotating elements 34 is applied to form a rotating element layer 184 asshown in FIG. 25. The thickness of the second layer of elastomer 186should be greater than the diameter of the rotating elements 34, butpreferably less than twice the diameter of the rotating elements 34. Auniform magnetic field 188 is applied in a direction normal to the layer185, 178 surfaces. The uniform magnetic field 188 will cause therotating elements 34 to seek out and align with the ferro-magneticelements 174 and possibly form strings of rotating elements such asstring 190. This is a well known effect and is the basis of the`magnetic brush` development systems used in xerography. At this pointthe compound structure of the rotating element layer 184 plus theferro-magnetic particle layer 178 can be cured. During the curingprocess, both layers 178, 186 will be bonded together.

After curing, any excess rotating elements 34 which have formed strings190 can easily be removed using a knife or by light rubbing because theywill be protruding from the cured elastomer 186. The compound structureof the cured rotating element layer 184 plus the cured ferro-magneticparticle layer 178 can be removed from the release layer 180 and swelledas in known in the art to produce gyricon sheet 46 shown in FIG. 23.

Hybrid Devices Containing Both Soft And Hard Magnetic Material

Two classes of improved gyricons and their operation have been describedthus far. Those containing soft magnetic material and those containinghard magnetic material. However, hybrid devices containing both types ofmagnetic material are also feasible.

Gyricon With Magnetic Trap And Opposed Latch

FIG. 26 shows a gyricon sheet 46 which has the magnetic elements of theembodiments shown in both FIG. 4 and FIG. 22, therefore the samereference numerals will be used to donate the same elements. In FIG. 26is shown a cross-section of a portion of a magnetically assisted Gyriconsheet 46. A black and white bichromal spherically or cylindricallysymmetric rotating element 34 is shown in which the black magnetizedsegment 40 is made from black pigments, some of which are permanentlymagnetizable. It should be noted that rotating element 320 shown in FIG.17b, could also be used interchangeably with the rotating element 34. Itshould also be noted that black and white are used here for illustrativepurposes only and any colors could be chosen. The white unmagnetizedsegment 38 is constructed from the usual materials and is notmagnetizable. This rotating element is contained in an oil filled cavity36. The soft magnetic material pad 42 is incorporated near the cavitystructure of each rotating element the same as shown in FIG. 4. Therotating element 34 is also made from materials that develop electricalpotentials in contact with the liquid in the oil filled cavity 36 and inthe presence of the electrical field, so that the segments 38, 40 of therotating element 34 develop different electrical potentials from eachother. Additionally, a magnetic pad 152 has been adhered to the surfaceof the gyricon sheet which is opposed to the soft magnetic material pad42. The magnetic pad 152 is constructed from a uniform permanentlymagnetized rubber sheet which has had etched areas 156 removed to createmagnetic hills 154 as discussed earlier with respect to FIG. 22. Inoperation, the device would work as a magnetic trap device describedhereinabove with additional stability applied from the soft magneticmaterial pad 42 for the condition when the rotating element 38 isdisposed away from and is being repelled by the magnetic pad. It shouldbe noted that this configuration may also use other of the rotatingelements discussed hereinabove including the rotating element with asingle pole magnetic segment, dual pole magnetic segments, or a rotatingelement with a single pole magnetic segment and a magnetic segment.

An alternative embodiment is shown in FIG. 27. FIG. 27 shows across-sectional view of a similar sheet using the same elements as usedin FIG. 26, except that the patterned magnetic pad 152 has been replacedwith a substantially uniform, thin soft magnetic material layer 210. Itshould also be noted that rotating element 320 shown in FIG. 17b can beused in this embodiment as well.

The soft magnetic layer 210 functions similarly to the soft magneticpads 42 discussed herein before. The magnetic segment 40 of the rotatingelement 34 induces a non-uniform magnetic attractive force between themagnetic segment 40 and the soft magnetic layer 210. This magnetic forcewill cause the rotating element 34 to be attached to the oil filledcavity 36 wall. The magnetic force will also require a larger electricalfield than otherwise to cause the rotating element 34 to start to rotatein the oil filled cavity 36 because the electrical field must firstovercome the magnetic force to cause the rotating element 34 to rotate.However, except for the increased value of the applied electric field,the gyricon sheet 46 may be addressed by any of the addressing methodsdescribed herein above or known in the art. Once the rotating element 34has rotated a short distance it will experience a much reduced forcefrom interaction with the soft magnetic material layer 210 and themotion of the rotating element 34 will be dominated by the appliedelectrical field.

Gyricon With Magnetic Trap And 90 Degree Latch

FIGS. 28 and 29 show a gyricon sheet 46 which has the magnetic elementsof the embodiments shown in FIG. 26, therefore the same referencenumerals will be used to denote the same elements. In FIG. 28 is shown across-section of a portion of a magnetically assisted Gyricon sheet 46.A spherical rotating element 200 is shown contained in an oil filledcavity 36. The rotating element 200 is different from the previousembodiments of rotating elements. The rotating element 200 has twotransparent end segments 202, 206 and a thin, colored central segment204 interposed between the two transparent end segments 202, 206.Additionally, the rotating element 200 has a polar permanently magneticsegment 208, of the same type as discussed hereinbefore adjacent to oneof the transparent end segments 206. The rotating element 200 providestwo optical states. The first is to display the colored central segmentto an observer as shown in FIG. 28. However, when the rotating element200, is rotated by 90 degrees, the colored central segment 204 is viewededge on and the rotating element 200 appears substantially transparentallowing backing sheet 212 to be viewed. Backing sheet 212 can be awhite, black, or colored or patterned sheet as known in the art. Gyricondevices utilizing rotating elements with transparent end segments andthin colored central segments are known in the art and a completedescription of their operation and uses is contained in U.S. Pat. No.5,717,514 by Sheridon titled "Polychromal Segmented Balls For A TwistingBall Display", and U.S. patent application Ser. No. 08/960,965 bySheridon et al. titled "Twisting Cylinder Display" incorporated byreference hereinabove.

A soft magnetic material pad 42 is incorporated near the cavitystructure of each rotating element as shown in FIGS. 28 and 29. Noticethat instead of being on the opposite side of the oil filled cavity 36from the magnetic pad 152, as shown in FIG. 26, the magnetic pad 42 isplace to one side of the oil filled cavity 36. This is to provide tworotational states, of the rotating element 200, which are 90 degreesfrom one another. The magnetic pad 152 is constructed from a uniformpermanently magnetized rubber sheet which has had etched areas 156removed to create magnetic hills 154 as discussed earlier.

In operation, the device would work as a magnetic trap device describedhereinabove with an additional rotational state supplied by the softmagnetic material pad 42. The polar magnetic segment 208 would interactwith either the magnetic pad 152 or the magnetic pad 42 to providemagnetic latching as discussed hereinabove. It should be noted that ifthe rotating element 200 is rotated from the magnetic pad 152 dockedposition it may not rotate in a direction that guarantees the polarmagnetic segment 208 is adjacent to the soft magnetic material pad 42.For this reason, it is probably advisable not to undergo a complete 90degree rotation when moving from the first state, shown in FIG. 28 tothe second state, shown in FIG. 29. A slightly lesser rotation willensure that the rotating element will rotate back to the first state inthe same direction it took in rotating from the first state. It shouldbe noted that the orientation of the polar magnetic segment 208 withrespect to rotating element 200 rotation is guaranteed by theorientation of the rotating element 200 in the electric field.

It should also be noted that rotating element 320 shown in FIG. 17b ismagnetically equivalent to rotating element 200 and could also be usedin a gyricon sheet constructed similarly with a 90 degree latch. Whilethe rotating element 200 provides two distinct states, one substantiallytransparent and one with a color, using the rotating element 320 wouldprovide three states, the two magnetic latching states as described withrespect to FIG. 28 plus the 90 degree latch state. The benefit toproviding the 90 degree latch is that the rotating element would display1/2 half of each color and the two unmagnetized segments 322,324. If theunmagnetized segments were, for instance, chosen to be black and white,the 90 degree latch state would provide 1/2 black and 1/2 white, orgrey.

An alternative embodiment is shown in FIGS. 30 and 31. FIGS. 30 and 31show a cross-sectional view of a similar sheet using the same elementsas used in FIGS. 28 and 29, except that the patterned magnetic pad 152has been replaced with a substantially uniform, thin soft magneticmaterial layer 210, as used hereinbefore with respect to FIG. 23 Thedevice works similarly to the device described hereinbefore with respectto FIG. 27.

The soft magnetic layer 210 functions similarly to the soft magneticpads 42 discussed herein before. The magnetic segment 208 of therotating element 200 induces a non-uniform magnetic attractive forcebetween the magnetic segment 208 and the soft magnetic layer 210. Thismagnetic force will cause the rotating element to be attached to the oilfilled cavity wall. The magnetic force will also require a largerelectrical field than otherwise to cause the rotating element 200 tostart to rotate in the oil filled cavity 36 because the electrical fieldmust first overcome the magnetic force to cause the rotating element 200to rotate. However, except for the increased value of the appliedelectric field, the gyricon sheet 46 may be addressed by any of theaddressing methods described herein above or known in the art. Once therotating element 200 has rotated a short distance it will experience amuch reduced force from interaction with the soft magnetic materiallayer 210 and the motion of the rotating element 200 will be dominatedby the applied electrical field. Thus the soft magnetic material pad 42and the soft magnetic material layer 210 provide two stable positionsfor the rotation of the rotating element 200 as the rotating element 200will be attracted to each of them when it is positioned such that thepolar magnetic segment 208 is adjacent to either the soft magneticmaterial pad 42 or the soft magnet material layer 210.

Rotating Element Fabrication

This rotating element can be fabricated as known in the art with amodified multiple rotating disk assembly, as described in U.S. Pat. No.5,717,514 by Sheridon titled "Polychromal Segmented Balls For A TwistingBall Display", and U.S. patent application Ser. No. 08/960,865 bySheridon et al. titled "Twisting Cylinder Display" and incorporated byreference hereinabove, or other planar stream/free jet type devices suchas those disclosed in U.S. Pat. No. 5,344,594, titled "Method ForFabrication Of Multicolored Balls For A Twisting Ball Display", bySheridon and also incorporated by reference hereinabove. Themanufacturing process and apparatus has been detailed thoroughlyhereinabove with respect to FIGS. 16 and 17. The manufacture of rotatingelement 200 requires the same 4 stream process as shown in FIGS. 16 and17 with one stream to be used for the polar magnetic segment, twostreams, one each, to be used for the two transparent end segments andone stream for the colored central segment.

Sheet Fabrication

The sheets may be fabricated using either Fabrication method 1 orFabrication Method 3 of the sheet manufacturing techniques discussedhereinabove with respect to Gyricons having a single latched state andshown in FIGS. 7 through 12. It should be noted however, that when astrong magnetic field is applied to orient the rotating elements in acommon direction, as shown hereinabove in FIGS. 7 and 12, that themagnetic field 108 should be oriented in a direction parallel to theplane of the uncured sheet 106 as shown in FIG. 32 when constructing thesheet with the 90 degree latch. The soft magnetic material layer may beprovided by using the technique discussed herein above with respect tosheet Fabrication method 2 and solidifying the layer before particlemigration has taken place. In all other respects, fabrication remainsthe same.

Unique Cylindrical Optical Gyricon Elements

All of the devices described thus far could have either cylindrical orspherical symmetry, but the magnetic modifications to controlthresholding and to create binary latching can be also applied to someunique cases of rotating cylinders to accomplish the same ends. Thereare some characteristics unique to these cylindrical cases that requireadditional discussion.

In Co-pending U.S. patent application Ser. No. 08/960,868 titled"Twisting Cylinder Display Using Multiple Chromatic Values" by Sheridon,and incorporated by reference hereinabove, cylindrically rotatingGyricon optical elements are disclosed. A distinct advantage of thesecylindrical elements is their ability to form a display capable of bothadditive color and good saturation of the basic colors. This isillustrated in FIG. 33 which also appears in U.S. patent applicationSer. No. 08/960,868. FIG. 33 shows a cross- sectional view of a gyriconsheet 220 made from a transparent optical medium 222 having an oilfilled cavity 224 enclosing a rotating element 226, as shown before.However, this rotating element 226 is comprised of a multisided displaysurface 230 encased in a transparent cylinder 228. In this embodiment,the multisided display surface 230 has three display surfaces 232, 234,236. Each display surface can be chosen to be a separate color, black,white or shades of grey to provide a gyricon sheet which can providegreyscale, highlight color or even a full-color RGB display. For thepurposes of this discussion, let us assume that the three displaysurfaces 232, 234, and 236 are selected to be red, blue, and greenrespectively to provide a full color RGB display. As in the previouscases, the element is addressed by providing an electric field (notshown) which causes the rotating element 226 to rotate to theorientation desired.

FIG. 34 shows the gyricon sheet 220 shown in FIG. 33 after modificationsto enable magnetic latching. As such, the same reference numerals willbe used to identify the same elements. In FIG. 34, the rotating elementhas been rotated to a position to allow an Observer O to view thedisplay surface 232, which is colored red. The rotating element 226 hasbeen modified such that each vertex of the multisided display surface230 has a magnetic portion resulting in three magnetic vertices 240,242, 244. The magnetic vertex is shown as a small cylinder in the cornerof each vertex of the multisided display element, however, this need notbe so. The entire vertex could be magnetic, or small magnetic portionsof other shapes could be included in the vertice. Additionally, whilethe entire vertices themselves could be made magnetic, the magneticvertices are depicted as a small magnetic portion contained within themultisided display surface to eliminate any interference with theviewable color on the display surfaces 232, 234, 236 by the magneticvertices 240, 242, 244. The magnetic vertices 240, 242, 244 are madefrom a magnetic material that has been permanently magnetized such asblack magnetic pigment Type 031182 by Wright Industries, Brooklyn, N.Y.as discussed herein before. The soft magnetic material pads 246, 248,250 are made from soft magnetic material, a magnetic material that iscapable of developing a strong magnetic dipole strength while exposed toa strong external magnetic field, but that is not capable of retainingsignificant remnant magnetism when the field is removed, as discussedherein before.

In operation, this embodiment of the gyricon sheet works similar toother embodiments discussed hereinabove. That is, when the magnetizedvertices 240, 242, 244 of the rotating element 226 are adjacent to thesoft magnetic material pads 246, 248, 250 embedded next to the oilfilled cavity 224, a strong magnetic force tends to hold the rotatingelement 226 in place. This is because the distance between themagnetized portions of the rotating element 226 and the soft magneticmaterial pads 246, 248, 250 is very short compared to the dimensions ofthe rotating element 226, thus providing a strong magnetic field. Thesoft magnetic material pads 246, 248, 250 are incorporated near the oilfilled cavity 224 of each rotating element 226 as shown and separatedfrom the oil filled cavity by a separation distance D_(s). For practicalpurposes, the separation distance D_(s) should be no more than thediameter d of the rotating element multiplied by a factor of three. Thesoft magnetic material pad 42 should preferably have a length l nosmaller than 1/4 of the rotating element diameter d. The onlyrestriction on the upper limit of the length l is that it must not be solarge as to interfere with surrounding rotational elements or their softmagnetic material pads. This will be dictated by the packing density ofthe gyricon sheet 46. Depending on the packing density, the length l ofthe soft magnetic material pad 42 can be as large as the diameter d ofthe rotating element 34 or even twice as large as the diameter d of therotating element 34 or more. This magnetic force will require a largerelectrical field than otherwise to cause the rotating element 226 tostart to rotate in the oil filled cavity 224. Once the rotating element226 has rotated a short distance it will experience a much reduced forcefrom interaction with the soft magnetic material pads 246, 248, 250 andthe motion of the rotating element 226 will be dominated by the appliedelectrical field.

It should be noted that the multisided display surface 230 need not belimited to three display surfaces 232, 234, 236 as shown in FIG. 34. Forinstance, FIG. 35 shows a modified version of the gyricon sheet 220shown in FIG. 34. As such, the same reference numerals will be used todenote the same elements. FIG. 35 shows a cross-sectional view of agyricon sheet 220 made from a transparent optical medium 222 having anoil filled cavity 224 enclosing a rotating element 260, as shown before.However, this rotating element 260 is comprised of a four sidedmultisided display surface 264 encased in a transparent cylinder 262. Inthis embodiment, the multisided display surface 230 has four displaysurfaces 266, 268, 270, 272. Each display surface can be chosen to be aseparate color, black, white or shades of grey to provide a gyriconsheet which can provide greyscale, highlight color or even a full-colorRGB display. For the purposes of this discussion, let us assume that thefour display surfaces 266, 268, 270, 272 are selected to be red, blue,and green and black respectively to provide a full color RGB display. Asin the previous cases, the element is addressed by providing an electricfield (not shown) which causes the rotating element 260 to rotate to theorientation desired.

In FIG. 35, the rotating element has been rotated to a position to allowan Observer O to view the display surface 266, which is colored red. Therotating element 260 has been modified such that each vertex of themultisided display surface 264 has a magnetic portion resulting in fourmagnetic vertices 274, 276, 278, 280. Each magnetic vertex is shown as asmall cylinder in the corner of each vertex of the multisided displaysurface 264, however, this need not be so. The entire vertex could bemagnetic, or small magnetic portions of other shapes could be includedin the vertex. Additionally, while the entire vertices themselves couldbe made magnetic, the magnetic vertices are depicted as a small magneticportion contained within the multisided display surface 264 to eliminateany interference with the viewable color on the display surfaces 266,268, 270, 272 by the magnetic vertices 274, 276, 278, 280. The magneticvertices 274, 276, 278, 280 are made from a magnetic material that hasbeen permanently magnetized such as black magnetic pigment Type 031182by Wright Industries, Brooklyn, N.Y. as discussed herein before. Thesoft magnetic material pads 282, 284, 286, 288 are made from softmagnetic material, a magnetic material that is capable of developing astrong magnetic dipole strength while exposed to a strong externalmagnetic field, but that is not capable of retaining significant remnantmagnetism when the field is removed, as discussed herein before. Thespacing and dimensions of the soft magnetic material pads 282, 284, 286,288 follow the same parameters as set forth in the discussionhereinbefore with respect to FIG. 34.

In operation, this embodiment of the gyricon sheet works similar toother embodiments discussed hereinabove. That is, when the magnetizedvertices 274, 276, 278, 280 of the rotating element 260 are adjacent tothe soft magnetic material pads 282, 284, 286, 288 embedded next to theoil filled cavity 224, a strong magnetic force tends to hold therotating element 260 in place. This is because the distance between themagnetized portions of the rotating element 260 and the soft magneticmaterial pads 282, 284, 286, 288 is short compared to the dimensions ofthe rotating element 260, thus providing a strong magnetic field. Thismagnetic force will require a larger electrical field than otherwise tocause the rotating element 260 to start to rotate in the oil filledcavity 224. Once the rotating element 260 has rotated a short distanceit will experience a much reduced force from interaction with the softmagnetic material pads 282, 284, 286, 288 and the motion of the rotatingelement 260 will be dominated by the applied electrical field.

In both FIGS. 34 and 35 the magnetic portions of the rotating elementshave been added at the vertices of the multisided display device. Itshould be noted that other arrangements are possible, for instance inthe centers of the faces of each of the display surfaces. However,positioning the magnetic portions and the corresponding soft magneticmaterial pads adjacent to the vertices of the multisided display surfacemaximizes the display area on a display surface because it minimizes theamount of the viewable display surface which must be used to providemagnetic latching.

Rotating Element Fabrication

The rotating element shown in FIGS. 34 and 35 can be manufactured usingknown drawing techniques such as those discussed in U.S. patentapplication Ser. No. 08/960,868 titled "A Twisting Cylinder DisplayUsing Multiple Chromatic Values" and incorporated by referencehereinabove. U.S. patent application Ser. No. 08/960,868 discloses thata large format display element could be constructed from glass orplastic. After assembly of the large format display element, one end ofthe large format display element is heated and a pulling device slowlydraws filaments from the large format display element. In order toconstruct the display elements shown in FIGS. 34 and 35, four thincylinders of magnetic material are added at the vertices. Because inthis cylindrical structure there are many elements fused together, itshould be understood that these components should mostly made from thesame base polymer, differently pigmented or dyed as appropriate. Thisinsures that all components of the cylinder have the sameviscosity/temperature relationship to allow the filaments to be "pulled"from a large format display element, as discussed in the U.S. patentapplication Ser. No. 08/960,868. Likewise, these magnetic rods shouldprobably also be made from the same base polymer, but with thesubstantial addition of magnetic pigment particles as discussedhereinbefore. When the filaments are being pulled from the large formatdisplay element they should be passed through a magnetic field, asdiscussed herein before and shown in U.S. patent application Ser. No.08/960,868 to insure that all display elements will be magnetized in thesame orientation.

Sheet Fabrication

The gyricon sheet 220 and the soft magnetic material pads 282, 284, 286,288 can be formed by mixing into an uncured elastomer soft magneticmaterial powder and magnetized rotating elements as discussed hereinabove with respect to FIG. 10. This is formed into an uncured sheet on asurface, and the curing is delayed to allow the pigment particles to beattracted to the magnetized vertices of the rotating elements. Thepigment particles will be attracted to the magnetized vertices becausethe magnetized vertices create a very non-uniform magnetic field intheir vicinities. This field provides the mechanical force to move thepigment particles to the surface of the magnetized vertices of therotating elements. When this process is sufficiently complete, a uniformmagnetic field is applied to the sheet, causing the rotating elementsand the attached soft magnetic material pigment particles, to rotateinto common alignment. as is discussed hereinabove with respect to FIG.12. While this field is continuously applied the elastomer sheet iscured as is known in the art. The sheet can then be swelled, as is alsoknown in the art.

It will be understood that the shape of the soft magnetic material pad42 will tend to conform to the shape of the rotating element 34 due tothe method of manufacture of the soft magnetic material pad 42. Forinstance, the soft magnetic material pad 42 may tend to curve slightlyand mimic the shape of the rotating element 34. Furthermore it will beunderstood that FIG. 34 shows a cross-sectional view for a cylindricallyshaped rotating element and the pad will tend to form in an elongatedshape for a cylindrically shaped rotating element.

Addressing Methods For Hard Magnetic Material Trap Devices

Two addressing methods exist for addressing the hard magnetic materialtrap devices described above. The first of these is to apply an electricfield of sufficient strength to overcome the magnetic fields holding therotating elements in place. Except for the increased value of theapplied electric field, the gyricon sheet can be addressed by any of theaddressing techniques described herein above or known in the art. Theadvantages of this method are simplicity in design of the addressingdevice, however the disadvantage lies in the greater strength of theelectric field that must be applied.

The second of these is to apply a small, localized magnetic field of theopposite polarity to negate the magnetic field holding a rotatingelement in place. This small, localized magnetic field will "unlock" aspecific rotating element, or a set of rotating elements, and allowrotation if an electric field of the correct polarity s concurrentlyapplied. While this method has the advantage that smaller strengthfields need to be generated it comes at the cost of added complexity inthe design of the addressing device.

The disclosure up to this point has focussed on using local magneticfields to provide greater stability of rotating elements in a gyriconsheet because the rotating elements can be latched into a desiredposition by using the local magnetic fields. The local magnetic fieldsprovide stability against inadvertent rotation of elements that mayoccur due to stray electrical fields when a gyricon sheet is, forinstance, handled in order to address the rotating elements and switchthem from one position to another an electric field is applied which isstrong enough to overcome the local magnetic field and cause therotating elements to rotate to a new desired state. However, the localmagnetic fields necessarily increase the strength of the electric fieldneeded to cause rotation of the elements. The most stable stored imageswill necessarily have the strongest local magnetic fields and requirethe strongest applied electrical fields to cause rotation of therotating elements to effect an image change. However, strongerelectrical fields require more bulky and expensive equipment to generateand, if strong enough, may present a possible hazard to the user.Therefore, it would be desirable to construct a gyricon sheet which usesstrong local magnetic fields for stability and a means of addressingsuch sheets-that uses only low to moderate strength electric fields toaddress the rotating elements within the sheets.

An addressing means that does not require the stronger electric fieldsneeded to overcome the strong local magnetic fields can be made by usingan external magnetic field which temporarily `unlocks` the rotatingelement in conjunction with a reduced electrical addressing field. Theexternal magnetic field will counteract the local magnetic fieldexperienced by the rotating element and effectively lowering themagnetic field experienced by the rotating element. The externalmagnetic field thereby permits lower electrical fields to be used foraddressing the rotating elements.

FIGS. 36 and 37 show a rotating element 300 trapped in an oil filledcavity 302. The rotating element 300 has two segments, a magneticsegment 306 and a non-magnetic segment 304. Additionally, a magnet 308having dimensions comparable to the rotating element 300 is disposednear the oil filled cavity 302. FIGS. 36 and 37 are consistent with thedescription of magnetic trapping devices discussed hereinabove withrespect to FIGS. 20-23 and could represent any of the configurationsdiscussed with respect to FIGS. 20-23. Additionally shown in FIGS. 36and 37 are dominant magnetic field lines 310 that exist between therotating element 300 and the magnet 308. In FIG. 36 the rotating element300 is trapped in the position where the magnetic segment 306 isproximate to the magnet 308. This is the position where the magneticsegment 306 and the magnet 308 are oriented so they attract each other.The field lines 310 are seen to connect between the two magnets,indicating their attraction for each other.

In FIG. 37 the rotating element 300 is trapped in the position where themagnetic segment 306 is repulsed by the magnetic 308. Here it can beseen that the dominant magnetic field lines 310 from the magneticsegment 306 and the dominant magnetic field lines of the magnet 308 tendto repel each other.

FIG. 38 shows the constructions of FIGS. 36 and 37 where an externalmagnetic field has been added in the form of a uniform and relativelylarge magnet 312. By large it is meant that the size of the magnet 312is much larger than the diameter of the rotating elements 300 by afactor of at least 10. Two sets of rotating elements 300 are depicted.The first set depicts the rotating elements 300 where the magneticsegment 306 is attracted to the magnet 308, as shown in FIG. 36. Thesecond set depicts the rotating elements 300 where the magnetic segment306 is repulsed by the magnetic 308 as depicted in FIG. 37. The dominantmagnetic field lines 310 are re-drawn to show the influence of thelarge, relatively uniform magnet 312. It should be pointed out thatwhile in this depiction the external magnetic field has been suppliedusing a magnet, other forms of supplying a magnetic field are known,such as an electric current changing linearly with time or a directcurrent in a coil. The means for supplying the external magnetic fieldis not important, only its size and magnitudes important.

Since both the rotating element 300 and the magnetic pad 308 are madepartially from magnetic material that has a high permeability, many ofthe magnetic field lines associated with the large, uniform magnet 312will bend to pass through and around these lesser magnets, as shown inFIG. 38. This effect causes the magnetic field associated with themagnet 312 to be non-uniform in the vicinity of both the rotatingelements 300 and their associated magnets 308, creating mechanicalforces that will be comparable to and oppositely acting to the forcesassociated with the magnetic fields 310 that exist between the rotatingelement 300 and the magnet 308 as shown in FIGS. 36 and 37. Effectively,this external magnetic field is nullifying the effects of the magneticfield associated with the magnet 308. The strength of the magnetic field310 created by the external magnet 312, or other magnetic fieldgenerating means as described above, decreases as the magnet 312 ispulled away from the rotating elements 300.

While the decreased magnetic field condition, as shown in FIG. 38exists, if an electrical field of less than sufficient strength toovercome the magnetic fields when the external magnetic field is notpresent is placed across the rotating elements 300 then the electricfield will cause the rotating elements 300 to rotate. The followingFigures illustrate how the magnetic unlocking mechanism operates for therotating element 300.

In FIG. 39, the magnet 312 is some distance away from rotating element300 and unable to supply a sufficient magnetic field to affect therotating element 300. The rotating element 300 is in the condition inwhich it is attracted to the magnet 308. The magnet 312 is approachingthe rotating element from the side opposite to the magnet 308.

FIG. 40 shows the magnet 312 being moved closer to the rotating element300. As the magnet 312 is moved closer to the rotating element 300, thegradient of its magnetic field in the vicinity of the rotating element300 increases to the point where it provides more attraction to therotating element 300 than the magnet 308. When this condition occurs,the rotating element 300 is lifted from the cavity wall and suspended inthe oil filled cavity 302.

FIG. 41 illustrates that when an external electrical field is appliedacross the rotating element 300 by using a charge differential 314 ofthe required polarity while the rotating element 300 is suspended in theoil filled cavity 302, the electric field will cause the rotatingelement 300 to rotate. When this happens, the rotating element 300 willalso move toward the upper cavity wall of the oil filled cavity 302.This can also happen if the magnet 312 is strong enough to lessen theeffect of the magnet 308, but not suspend rotating element 300 withinthe cavity. In that case, the rotating element 300 will still be incontact with the wall of the oil filled cavity 302, as illustrated inFIG. 39, but will rotate and move towards the upper cavity wall of theoil filled cavity 302 when the electric field is applied.

FIG. 42 shows the removal of magnet 312. As the magnet 312 is pulledaway from the rotating element 300, the rotating element 300 will comeunder the magnetic dominance of the magnet 308 once again, locking itinto its new state. The rotating element 300 is now in the orientationwhere it is repelled by the magnet 308.

If the electrical addressing field were not present, the rotatingelement 300 would not have rotated and upon removal of the magnet 312the rotating element 300 would simply have returned to its initialcondition, as shown in FIG. 39.

In FIG. 43, the magnet 312 is again depicted some distance away fromrotating element 300 and unable to supply a sufficient magnetic field toaffect the rotating element 300. However, the rotating element 300 is inthe orientation in which it is repelled from the magnet 308. The magnet312 is approaching the rotating element from the side opposite to themagnet 308.

FIG. 44 depicts the magnet 312 moving closer to the rotating element300. The increasing magnetic field from the magnet 312 will eventuallycause the rotating element 300 to be repulsed from the upper wall of theoil filled cavity 302 and be suspended in the oil filled cavity 302. Theforce of the magnetic field from the magnet 312 also causes the rotatingelement 300 to begin to rotate.

FIG. 45 shows further movement of the rotating element 300 in the oilfilled cavity 302 when the magnet 312 is kept in proximity to therotating element 300. The force of the magnetic field from the magnet312 continues to cause the rotating element 300 to rotate.

FIG. 46 shows the result of the rotation. The rotating element 300 endsup in the stable condition where it is attracted to the magnet 308. Whenthe magnet 312 is removed the rotating element 312 will remain locked inplace under the influence of the magnetic field from the magnet 308.

It should be noted that in the sequence depicted in FIGS. 43-46, therotating element 300 rotated without the presence of an electric field.This shows that a magnetic field alone can be used to set all therotating elements 300 to the same orientation or to clear the gyricondisplay of a displayed image. Had an electric field been present acrossthe rotating element 300 the rotation of the rotating element shown inFIGS. 44-46 could have been prevented. However, this is an inherentlyunstable condition, and preventing rotating element 300 rotation wouldhave required either a large electrical field as the magnet 312 wasmoved closer to the rotating element, or precisely controlled magneticstrengths of the magnet 308, magnetic segment 306 of the rotatingelement 300 and magnet 312.

FIGS. 47-51 apply the concepts shown in FIGS. 39-46 to show howaddressing with a magnetic unlocking device in conjunction with anelectrical field work on a gyricon sheet. Because the model and elementsused in FIGS. 39-46 is identical to the model used in FIGS. 39-46 thesame reference numerals will be used for the same elements, however theletter "a", "b", "c", or "d" will be appended to the reference numeralsto denote a particular rotating element and all the elements associatedwith that particular rotating element for clarity.

FIG. 47 shows four rotating elements 300a, 300b, 300c, 300d. Rotatingelements 300a and 300b are oriented in the state where they areattracted to their magnets 308a and 308b respectively. Rotating elements300c and 300d are oriented in the state where they are repelled by theirmagnets 308c and 308d, respectively. A pair of electrodes 316a, 316b,316c, 316d are associated with each rotating element 300a, 300b, 300c,300d to represent the application of an electric field across therotating elements 300a, 300b, 300c, 300d. An electric field exists whena pair of electrodes 316b has a charge differential 314b placed acrossit. While an electric field may be applied across a pair of electrodes316b using a charge differential 314b, other methods are also known andmay be used such as image wise charge placement on the surface of asheet. The magnet 312 is moving closer to the rotating elements 300a,300b, 300c, 300d but is sufficiently distant from the rotating elements300a, 300b, 300c, 300d that the magnetic fields from the magnet 312 haveno impact on the rotating elements 300a, 300b, 300c, 300d. The chargedifferential 314b on the electrodes 316b and its associated electricfield is also too small to cause rotation of the rotating element 300b.

In FIG. 48, the magnet 312 has moved sufficiently close to the rotatingelements 300a, 300b, 300c, 300d that its associated magnetic fieldprovides the dominant effect on the rotating elements 300a, 300b, 300c,300d. Rotating elements 300a, and 300b are attracted to the magnet 312and are suspended in their oil filled cavities 302a, 302b. Rotatingelements 300c and 300d are repelled from the magnet 312 and are alsosuspended in their oil filled cavities 302c and 302d Additionally, themagnetic field associated with the magnet 312 has started to inducerotation in rotating elements 300c and 300d. The charge differential314b across electrodes 316b and the associated electric field is stillineffectual in inducing rotation in rotating element 300b because themagnetic field from magnet 312 is the dominant force.

FIG. 49 shows the return of a steady state condition after the rotationof rotating elements 300c and 300d showing that all the rotatingelements 300a, 300b, 300c, and 300d are in the same orientation. Themagnetic field from magnet 312 is still the dominating force.Additionally, a charge differential 314d has been placed acrosselectrodes 316d to apply and electric field across rotating element300d. If it is desired that any element be in the opposite state then anelectric field must be placed across that element before the magnet 312is removed to cause rotation of that rotating element. It should benoted that the electric field could have been placed earlier in theprocess with no harmful effects as has been illustrated with element300b.

FIG. 50 shows the magnet 312 being moved away from the rotating elements300a, 300b, 300c, and 300d. As the magnet 312 is moved away from therotating elements 300a, 300b, 300c, 300d, the effects of its magneticfield on the rotating elements 300a, 300b, 300c, 300d will decrease. Asit is further withdrawn the magnet 312 and the magnets 308a, 308b, 308c,308d will exert equal and nearly opposite forces on the ball. Underthese conditions the electric field exerted by the charge differential314b, 314d on the electrodes 316b, 316d will cause the associatedrotating elements 300b, 300d to rotate.

FIG. 51 shows the stable state of the rotating elements 300a, 300b,300c, and 300d after the magnet 312 has been completely withdrawn.Rotating elements 300a, 300c and now in the orientation where therotating elements 300a, 300c are attracted to their associated magnets308a, 308c respectively and rotating elements 300b, 300d are in theorientation where the rotating elements 300b, 300d are repelled by theirassociated magnets 308b, 308d respectively.

As can be seen from the above sequence, there is a great deal of leewayin how the addressing steps are performed. If no electric field isplaced across any of the rotating elements while the magnet 312 isbrought into proximity of the rotating elements, then all the elementswill be changed to and remain in the same state. Therefore, if only"erasure" of the image is required then no electric fields are required.Any electric field that is desired to cause rotating elements to rotatemay be placed across those rotating elements during any time in theprocess from before the magnet 312 is brought into proximity to therotating elements up to the point where the magnet 312 is about to movedaway from the rotating elements. So long as the electric field is inplace prior to removal of the "unlocking" magnet field of the magnet 312then the rotating elements will be rotated. The addressing sequence canalso be broken down into two steps, the first being the approach of themagnet 312 with no applied electric fields to effect "erasure" of anyimage followed by the rapport of the magnet 312 with applied electricfields to effect "writing" of a new image.

With this method of addressing the rotating elements there are minimalrequirements on the accuracy of the strengths of the magnets used in thesystem (magnets 308, magnet 312 and magnetic segments 306 of rotatingelements 300) and for careful placement of the magnet 312. Simply movingmagnet 312 close enough to rotating elements 300 to create an excessmagnetic field and then moving it away suffices to satisfy the systemrequirements.

The above sequence illustrates a magnet 312 of a "north" polarityapproaching the rotating elements from one side only. It should be notedthat the same sequence can be implemented using a magnet of the oppositepolarity so long as it also approaches the rotating elements from theopposite side.

This concept shown above may be implemented in several ways. Forinstance, any of the methods for providing and electric field andaddressing a gyricon sheet, which are already known, such as thosediscussed hereinabove, may be combined with either a sheet magnet or ascanning magnet of sufficient strength. A scanning addressing system canbe made by combining previously known scanning addressing systems, suchas that discussed in U.S. Pat. No. 5,389,945, by Sheridon titled"Writing System Including Paper-Like Digitally Addressed Media andAddressing Device Therefor" and incorporated by reference hereinabove,with a magnet to be scanned as well. The magnetic field can be createdby using a permanent magnet, or an electromagnet energized by a currentflowing through a coil or any other method form creating a magneticfield.

What is claimed is:
 1. A method of making rotating element sheetmaterial comprising:a) providing a sheet of sheet material comprising asubstrate with a surface and at least one rotatable element disposedtherein, said element having an optical and electrical anisotropy, andcomprising at least two portions, wherein at least one portion ismagnetized, b) orienting the at least one element such that the at leastone magnetized portion is disposed towards the surface, c) applying alayer of hardenable mixture containing a plurality of magnetic particlesto the surface, d) migrating at least a portion of the plurality ofmagnetic particles to the area of the layer in the vicinity of themagnetized portion of the rotatable element, e) solidifying said layerto trap at least a portion of the plurality of magnetic particles insaid layer in the area of the layer in the vicinity of the magnetizedportion of the rotatable element to form a magnetic pad.
 2. The methodof claim 1 wherein the step of orienting the at least one rotatableelement comprises applying a substantially uniform electric field. 3.The method of claim 1 wherein the magnetic particles comprise softmagnetic material.
 4. The method of claim 3 wherein the soft magneticmaterial comprises magnetic pigment.
 5. The method of claim 4 whereinthe magnetic pigment comprises black magnetic pigment.
 6. The method ofclaim 1 wherein the magnetic particles comprise hard magnetic material.7. The method of claim 6 wherein the hard magnetic material comprisesmagnetic pigment.
 8. The method of claim 7 wherein the magnetic pigmentcomprises black magnetic pigment.
 9. The method of claim 1 wherein thehardenable liquid comprises uncured elastomer.
 10. The method of claim 9wherein the step of solidifying comprises curing the elastomer.
 11. Themethod of claim 1 wherein the hardenable liquid comprises epoxy.
 12. Themethod of claim 1 wherein the hardenable liquid comprises a moltenpolymer.
 13. The method of claim 1 wherein the step of providing a sheetof sheet material comprisesa. providing a sheet of sheet materialcomprising a substrate with a surface and at least one rotatable elementdisposed therein, said element having an optical and electricalanisotropy, and comprising at least two portions, wherein at least oneportion is magnetizable, b. orienting the at least one element such thatthe at least one magnetizable portion is disposed towards the surface,and c. magnetizing the magnetizable portion.
 14. The method of claim 13wherein the step of orienting the at least one element comprisesapplying a substantially uniform electric field.
 15. The method of claim13 wherein the step of magnetizing the magnetizable portion comprisesapplying a substantially uniform magnetic field.
 16. The method of claim15 wherein the substantially uniform magnetic field is appliedtransverse to the surface of the substrate.
 17. The method of claim 1wherein the step of providing a sheet of sheet material comprisesproviding a sheet of sheet material comprising a substrate with asurface and at least one rotatable element disposed therein, saidelement having an optical and electrical anisotropy, and comprising atleast two portions, wherein at least one portion is magnetized, and atleast one magnetic material pad disposed within said substrateassociated with an element, spaced from the associated element and soconstructed and arranged such that when the element is oriented suchthat the magnetized portion of the element is the portion of the elementnearest the magnetic material pad a magnetic attractive force will existbetween the magnetic material pad and the at least one magnetizedportion.